graft: give helpful warning for empty grafts

It was just showing a status message with the internal revision number.
Instead, show a warning like

note: graft of 27:3aaa8b6725f0 "28" created no changes to commit

(message tweaked in-flight by mpm)

mercurial/commands.py

@@ -3495,7 +3495,8 @@ def graft(ui, repo, *revs, **opts):
             node = repo.commit(text=message, user=user,
                         date=date, extra=extra, editor=editor)
             if node is None:
-                ui.status(_('graft for revision %s is empty\n') % ctx.rev())
+                ui.warn(_('note: graft of %s created no changes to commit\n') %
+                        desc)
     finally:
         wlock.release()
 

tests/test-graft.t

@@ -717,3 +717,4 @@ Empty graft
   $ hg tag -f something
   $ hg graft -qr 27
   $ hg graft -qf 27
+  note: graft of 27:3aaa8b6725f0 "28" created no changes to commit