added SpecifiedBranchCommit with git-merge command

README.md

@@ -112,7 +112,10 @@ affectedModuleDetector {
  - `compareFrom`: A commit to compare the branch changes against. Can be either:
     - PreviousCommit: compare against the previous commit
     - ForkCommit: compare against the commit the branch was forked from
-    - SpecifiedBranchCommit: specify the branch to compare changes against using the `specifiedBranch` configuration before the `compareFrom` configuration
+    - SpecifiedBranchCommit: compare against the last commit of `$specifiedBranch` using `git rev-parse` approach. 
+    - SpecifiedBranchCommit2: compare against the nearest ancestors with `$specifiedBranch` using `git merge base` approach.
+
+ **Note:** specify the branch to compare changes against using the `specifiedBranch` configuration before the `compareFrom` configuration
  - `excludedModules`: A list of modules that will be excluded from the build process
  - `includeUncommitted`: If uncommitted files should be considered affected
  - `top`: The top of the git log to use. Must be used in combination with configuration `includeUncommitted = false`

affectedmoduledetector/src/main/kotlin/com/dropbox/affectedmoduledetector/commitshaproviders/CommitShaProvider.kt

@@ -4,6 +4,7 @@ import com.dropbox.affectedmoduledetector.GitClient
 import com.dropbox.affectedmoduledetector.Sha
 
 interface CommitShaProvider {
+
     fun get(commandRunner: GitClient.CommandRunner): Sha
 
     companion object {
@@ -17,9 +18,14 @@ interface CommitShaProvider {
                     }
                     SpecifiedBranchCommit(specifiedBranch)
                 }
+                "SpecifiedBranchCommit2" -> {
+                    requireNotNull(specifiedBranch) {
+                        "Specified branch must be defined"
+                    }
+                    SpecifiedBranchCommit2(specifiedBranch)
+                }
                 else -> throw IllegalArgumentException("Unsupported compareFrom type")
             }
         }
     }
 }
-

affectedmoduledetector/src/main/kotlin/com/dropbox/affectedmoduledetector/commitshaproviders/SpecifiedBranchCommit2.kt

@@ -0,0 +1,17 @@
+package com.dropbox.affectedmoduledetector.commitshaproviders
+
+import com.dropbox.affectedmoduledetector.GitClient
+import com.dropbox.affectedmoduledetector.Sha
+
+class SpecifiedBranchCommit2(private val branch: String) : CommitShaProvider {
+
+    override fun get(commandRunner: GitClient.CommandRunner): Sha {
+        val currentBranch = commandRunner.executeAndParseFirst(CURRENT_BRANCH_CMD)
+        return commandRunner.executeAndParseFirst("git merge-base $currentBranch $branch")
+    }
+
+    companion object {
+
+        const val CURRENT_BRANCH_CMD = "git rev-parse --abbrev-ref HEAD"
+    }
+}