Handle PRs not based off recent main in large files check (#26420)

Previously failed due to trying to go all the way back to what Github
calls the "base" SHA, which turns out is what we'll merge on top of
rather than the parent of the first commit in the PR.

Follow up to #26371.

[trivial fix, not reviewed]

.github/workflows/CI.yml

@@ -153,14 +153,15 @@ jobs:
           baseSHA=${{github.event.pull_request.base.sha}}
           headSHA=${{github.event.pull_request.head.sha}}
 
+          echo "Base SHA: $baseSHA"
+          echo "Head SHA: $headSHA"
+          echo "${{github.event.pull_request.commits}} commits in PR"
+
           # Loop backward through commits added in the PR, starting from the
           # latest.
+          commitIdx=0
           for commit in $(git rev-list $baseSHA..$headSHA)
           do
-            if [ "$commit" = "$baseSHA" ]; then
-              break
-            fi
-
             echo "Checking commit: $commit"
             git checkout -q $commit
 
@@ -180,4 +181,11 @@ jobs:
                 exit 1
               fi
             done <<< "$newFiles"
+
+            # Stop after going through the number of commits this PR has.
+            commitIdx=$((commitIdx+1))
+            if [ "$commitIdx" =  "${{github.event.pull_request.commits}}" ]; then
+              echo "Stopping after $commitIdx commits, on commit $commit"
+              break
+            fi
           done