cmd/coordinator: correctly thread resolving comments

Third time's the charm.

For golang/go#39828.

Change-Id: I3eeb17b3c64bbcf27d2ce5e1a4a9207d5daf50d8
Reviewed-on: https://go-review.googlesource.com/c/build/+/318129
Trust: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

cmd/coordinator/coordinator.go

@@ -1511,6 +1511,7 @@ func (ts *trySet) notifyStarting() {
 			}
 			unresolved := false
 			ri.Comments["/PATCHSET_LEVEL"] = append(ri.Comments["/PATCHSET_LEVEL"], gerrit.CommentInput{
+				InReplyTo:  t.root.ID,
 				Message:    "Superseded.",
 				Unresolved: &unresolved,
 			})