diff --git a/action.yml b/action.yml
index d81583f..893c4b2 100644
--- a/action.yml
+++ b/action.yml
@@ -6,9 +6,14 @@ inputs:
   token:
     description: 'GitHub API token with read permissions for pull requests in this repository.'
     required: true
+  fetch-depth:
+    description: |
+      Number of commits to fetch each time we need to deepen git history in order to find the
+      merge base between the head ref and the base ref of the pull request
+    default: 10
   git-diff-args:
     description: |
-      Optional custom arguments to forward to `git diff` when retrieving the diff of the pull request that triggered this workflow.
+      Custom arguments to forward to `git diff` when retrieving the diff of the pull request that triggered this workflow.
       The result of that `git diff` command will be used for evaluation with sizeup.
     default: '--ignore-space-change'
   configuration-file-path:
diff --git a/badges/coverage.svg b/badges/coverage.svg
index f7c1988..bef488a 100644
--- a/badges/coverage.svg
+++ b/badges/coverage.svg
@@ -1 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="116" height="20" role="img" aria-label="Coverage: 61.59%"><title>Coverage: 61.59%</title><linearGradient id="s" x2="0" y2="100%"><stop offset="0" stop-color="#bbb" stop-opacity=".1"/><stop offset="1" stop-opacity=".1"/></linearGradient><clipPath id="r"><rect width="116" height="20" rx="3" fill="#fff"/></clipPath><g clip-path="url(#r)"><rect width="63" height="20" fill="#555"/><rect x="63" width="53" height="20" fill="#e05d44"/><rect width="116" height="20" fill="url(#s)"/></g><g fill="#fff" text-anchor="middle" font-family="Verdana,Geneva,DejaVu Sans,sans-serif" text-rendering="geometricPrecision" font-size="110"><text aria-hidden="true" x="325" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="530">Coverage</text><text x="325" y="140" transform="scale(.1)" fill="#fff" textLength="530">Coverage</text><text aria-hidden="true" x="885" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="430">61.59%</text><text x="885" y="140" transform="scale(.1)" fill="#fff" textLength="430">61.59%</text></g></svg>
\ No newline at end of file
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="116" height="20" role="img" aria-label="Coverage: 56.29%"><title>Coverage: 56.29%</title><linearGradient id="s" x2="0" y2="100%"><stop offset="0" stop-color="#bbb" stop-opacity=".1"/><stop offset="1" stop-opacity=".1"/></linearGradient><clipPath id="r"><rect width="116" height="20" rx="3" fill="#fff"/></clipPath><g clip-path="url(#r)"><rect width="63" height="20" fill="#555"/><rect x="63" width="53" height="20" fill="#e05d44"/><rect width="116" height="20" fill="url(#s)"/></g><g fill="#fff" text-anchor="middle" font-family="Verdana,Geneva,DejaVu Sans,sans-serif" text-rendering="geometricPrecision" font-size="110"><text aria-hidden="true" x="325" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="530">Coverage</text><text x="325" y="140" transform="scale(.1)" fill="#fff" textLength="530">Coverage</text><text aria-hidden="true" x="885" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="430">56.29%</text><text x="885" y="140" transform="scale(.1)" fill="#fff" textLength="430">56.29%</text></g></svg>
\ No newline at end of file
diff --git a/dist/index.js b/dist/index.js
index 9ad1763..fa63db5 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -16382,39 +16382,51 @@ exports.loadConfiguration = loadConfiguration;
  */
 async function fetchDiff(pull) {
     const git = (0, simple_git_1.simpleGit)('.', { trimmed: true });
-    const diffArgs = ['--merge-base', `origin/${pull.base.ref}`].concat(core.getInput('git-diff-args').split(/\s+/));
-    core.info(`Retrieving diff with \`git diff ${diffArgs.join(' ')}\``);
-    // Fetch all commits for the head branch back to where it diverged from the base.
-    core.debug(`Fetching ${pull.commits + 1} commits for ${pull.head.ref}`);
+    const fetchDepth = parseInt(core.getInput('fetch-depth'));
+    core.debug(`Fetching base ref "${pull.base.ref}" and head ref "${pull.head.ref}" with depth ${fetchDepth}`);
     await git.fetch([
         'origin',
+        `+${pull.base.ref}:${pull.base.ref}`,
         `+${pull.head.ref}:${pull.head.ref}`,
-        `--depth=${pull.commits + 1}`,
+        `--depth=${fetchDepth}`,
         '--no-tags',
         '--prune',
         '--no-recurse-submodules'
     ]);
-    core.debug(`Switching to branch ${pull.head.ref}`);
+    core.debug(`Switching to head ref "${pull.head.ref}"`);
     await git.raw('switch', pull.head.ref);
-    // Fetch commits for the base branch that were made since the head diverged from it.
-    const divergedFrom = await git.raw('rev-list', '--max-parents=0', 'HEAD');
-    const divergedAt = await git.show([
-        '--quiet',
-        '--date=unix',
-        '--format=%cd',
-        divergedFrom
-    ]);
-    core.debug(`Retrieving history for origin/${pull.base.ref} since ${divergedFrom} which was committed at ${divergedAt}`);
-    await git.fetch([
-        'origin',
-        `+${pull.base.ref}:${pull.base.ref}`,
-        `--shallow-since=${divergedAt}`,
-        '--no-tags',
-        '--prune',
-        '--no-recurse-submodules'
-    ]);
-    // Now we have all relevant history from both base and head branches, so we
-    // can compute an accurate diff relative to the base branch.
+    core.info(`Searching for merge base between "${pull.base.ref}" and "${pull.head.ref}"`);
+    let mergeBase = '';
+    let fetches = 0;
+    while (mergeBase) {
+        try {
+            mergeBase = await git.raw('merge-base', pull.base.ref, pull.head.ref);
+            core.info(`Found merge base: ${mergeBase}`);
+        }
+        catch (e) {
+            if (fetches === 0) {
+                core.debug(`git merge-base error: ${e.message}`);
+                core.info('Deepening git history in search of merge base');
+            }
+            else if (fetches % 10 === 0) {
+                core.debug(`git merge-base error: ${e.message}`);
+                core.info(`Still searching for merge base after ${fetches} fetches`);
+            }
+            fetches++;
+            await git.fetch([
+                'origin',
+                pull.base.ref,
+                pull.head.ref,
+                `--deepen=${fetchDepth}`,
+                '--no-tags',
+                '--prune',
+                '--no-recurse-modules'
+            ]);
+        }
+    }
+    core.debug(`Performed ${fetches} fetch${fetches === 1 ? '' : 'es'} to find the merge base`);
+    const diffArgs = ['--merge-base', pull.base.ref].concat(core.getInput('git-diff-args').split(/\s+/));
+    core.info(`Retrieving diff with \`git diff ${diffArgs.join(' ')}\``);
     return git.diff(diffArgs);
 }
 exports.fetchDiff = fetchDiff;
diff --git a/src/initializer.ts b/src/initializer.ts
index 34e1992..9968bb2 100644
--- a/src/initializer.ts
+++ b/src/initializer.ts
@@ -35,48 +35,65 @@ export function loadConfiguration(): Configuration {
  */
 export async function fetchDiff(pull: PullRequest): Promise<string> {
   const git = simpleGit('.', { trimmed: true })
-  const diffArgs = ['--merge-base', `origin/${pull.base.ref}`].concat(
-    core.getInput('git-diff-args').split(/\s+/)
-  )
+  const fetchDepth = parseInt(core.getInput('fetch-depth'))
 
-  core.info(`Retrieving diff with \`git diff ${diffArgs.join(' ')}\``)
+  core.debug(
+    `Fetching base ref "${pull.base.ref}" and head ref "${pull.head.ref}" with depth ${fetchDepth}`
+  )
 
-  // Fetch all commits for the head branch back to where it diverged from the base.
-  core.debug(`Fetching ${pull.commits + 1} commits for ${pull.head.ref}`)
   await git.fetch([
     'origin',
+    `+${pull.base.ref}:${pull.base.ref}`,
     `+${pull.head.ref}:${pull.head.ref}`,
-    `--depth=${pull.commits + 1}`,
+    `--depth=${fetchDepth}`,
     '--no-tags',
     '--prune',
     '--no-recurse-submodules'
   ])
 
-  core.debug(`Switching to branch ${pull.head.ref}`)
+  core.debug(`Switching to head ref "${pull.head.ref}"`)
   await git.raw('switch', pull.head.ref)
 
-  // Fetch commits for the base branch that were made since the head diverged from it.
-  const divergedFrom = await git.raw('rev-list', '--max-parents=0', 'HEAD')
-  const divergedAt = await git.show([
-    '--quiet',
-    '--date=unix',
-    '--format=%cd',
-    divergedFrom
-  ])
+  core.info(
+    `Searching for merge base between "${pull.base.ref}" and "${pull.head.ref}"`
+  )
+  let mergeBase = ''
+  let fetches = 0
+  while (mergeBase) {
+    try {
+      mergeBase = await git.raw('merge-base', pull.base.ref, pull.head.ref)
+      core.info(`Found merge base: ${mergeBase}`)
+    } catch (e) {
+      if (fetches === 0) {
+        core.debug(`git merge-base error: ${(e as Error).message}`)
+        core.info('Deepening git history in search of merge base')
+      } else if (fetches % 10 === 0) {
+        core.debug(`git merge-base error: ${(e as Error).message}`)
+        core.info(`Still searching for merge base after ${fetches} fetches`)
+      }
+      fetches++
+      await git.fetch([
+        'origin',
+        pull.base.ref,
+        pull.head.ref,
+        `--deepen=${fetchDepth}`,
+        '--no-tags',
+        '--prune',
+        '--no-recurse-modules'
+      ])
+    }
+  }
+
   core.debug(
-    `Retrieving history for origin/${pull.base.ref} since ${divergedFrom} which was committed at ${divergedAt}`
+    `Performed ${fetches} fetch${
+      fetches === 1 ? '' : 'es'
+    } to find the merge base`
   )
-  await git.fetch([
-    'origin',
-    `+${pull.base.ref}:${pull.base.ref}`,
-    `--shallow-since=${divergedAt}`,
-    '--no-tags',
-    '--prune',
-    '--no-recurse-submodules'
-  ])
 
-  // Now we have all relevant history from both base and head branches, so we
-  // can compute an accurate diff relative to the base branch.
+  const diffArgs = ['--merge-base', pull.base.ref].concat(
+    core.getInput('git-diff-args').split(/\s+/)
+  )
+  core.info(`Retrieving diff with \`git diff ${diffArgs.join(' ')}\``)
   return git.diff(diffArgs)
 }