Fix GetCommitBranchStart bug (#33298)

Fix #33265
Fix #33370

This PR also fixes some bugs in `TestGitGeneral`.

modules/git/commit_test.go

@@ -357,5 +357,5 @@ func Test_GetCommitBranchStart(t *testing.T) {
 	startCommitID, err := repo.GetCommitBranchStart(os.Environ(), "branch1", commit.ID.String())
 	assert.NoError(t, err)
 	assert.NotEmpty(t, startCommitID)
-	assert.EqualValues(t, "9c9aef8dd84e02bc7ec12641deb4c930a7c30185", startCommitID)
+	assert.EqualValues(t, "95bb4d39648ee7e325106df01a621c530863a653", startCommitID)
 }

modules/git/repo_commit.go

@@ -519,6 +519,7 @@ func (repo *Repository) AddLastCommitCache(cacheKey, fullName, sha string) error
 	return nil
 }
 
+// GetCommitBranchStart returns the commit where the branch diverged
 func (repo *Repository) GetCommitBranchStart(env []string, branch, endCommitID string) (string, error) {
 	cmd := NewCommand(repo.Ctx, "log", prettyLogFormat)
 	cmd.AddDynamicArguments(endCommitID)
@@ -533,7 +534,8 @@ func (repo *Repository) GetCommitBranchStart(env []string, branch, endCommitID s
 
 	parts := bytes.Split(bytes.TrimSpace(stdout), []byte{'\n'})
 
-	var startCommitID string
+	// check the commits one by one until we find a commit contained by another branch
+	// and we think this commit is the divergence point
 	for _, commitID := range parts {
 		branches, err := repo.getBranches(env, string(commitID), 2)
 		if err != nil {
@@ -541,11 +543,9 @@ func (repo *Repository) GetCommitBranchStart(env []string, branch, endCommitID s
 		}
 		for _, b := range branches {
 			if b != branch {
-				return startCommitID, nil
+				return string(commitID), nil
 			}
 		}
-
-		startCommitID = string(commitID)
 	}
 
 	return "", nil