Git 2.28 no longer permits diff with ... on unrelated branches (#12364)

* Git 2.28 no longer permits diff with ... on unrelated branches

Signed-off-by: Andrew Thornton <art27@cantab.net>

* need to check stderr

services/gitdiff/gitdiff.go

@@ -752,6 +752,12 @@ func GetDiffRangeWithWhitespaceBehavior(repoPath, beforeCommitID, afterCommitID
 		shortstatArgs = []string{git.EmptyTreeSHA, afterCommitID}
 	}
 	diff.NumFiles, diff.TotalAddition, diff.TotalDeletion, err = git.GetDiffShortStat(repoPath, shortstatArgs...)
+	if err != nil && strings.Contains(err.Error(), "no merge base") {
+		// git >= 2.28 now returns an error if base and head have become unrelated.
+		// previously it would return the results of git diff --shortstat base head so let's try that...
+		shortstatArgs = []string{beforeCommitID, afterCommitID}
+		diff.NumFiles, diff.TotalAddition, diff.TotalDeletion, err = git.GetDiffShortStat(repoPath, shortstatArgs...)
+	}
 	if err != nil {
 		return nil, err
 	}