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!
@@ -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")