graft: show hashes in user-facing messages Graft was in various places just showing the revision number in status messges. Instead, also show the stable and more useful short hash.

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)