Submitting review ec1a0bb

Check that parent comment exists before creating a comment
A proposed fix to issue #25

Would be nice to be able to instead use the `commentsByHash` that is already computed in [`review.loadComments`](https://github.com/google/git-appraise/blob/master/review/review.go#L177). Suggestions welcome!

Author: ojarjur

commands/comment.go

@@ -40,6 +40,18 @@ var (
 	commentNmw     = commentFlagSet.Bool("nmw", false, "'Needs More Work'. Set this to express your disapproval. This cannot be combined with lgtm")
 )
 
+func commentHashExists(hashToFind string, threads []review.CommentThread) bool {
+	for _, thread := range threads {
+		if thread.Hash == hashToFind {
+			return true
+		}
+		if commentHashExists(hashToFind, thread.Children) {
+			return true
+		}
+	}
+	return false
+}
+
 // commentOnReview adds a comment to the current code review.
 func commentOnReview(repo repository.Repo, args []string) error {
 	commentFlagSet.Parse(args)
@@ -100,6 +112,10 @@ func commentOnReview(repo repository.Repo, args []string) error {
 		return errors.New("There is no matching review.")
 	}
 
+	if *commentParent != "" && !commentHashExists(*commentParent, r.Comments) {
+		return errors.New("There is no matching parent comment.")
+	}
+
 	commentedUponCommit, err := r.GetHeadCommit()
 	if err != nil {
 		return err