shelve: status messages from unshelve
It was hard for the user to know what was going on when unshelving - especially
if the user had to resolve conflicts and thus got to see the intermediate
states.
Seeing that pending changes was gone could scare the user, make him panic, and
do stuff that really made him lose data.
Merging (both when rebasing and with pending changes) also requires some
understanding of where in the process you are and what you are merging.
To help the user we now show a couple of status messages (when relevant):
temporarily committing pending changes (restore with 'hg unshelve --abort')
rebasing shelved changes