commit: if relevant, tell user their commit message was saved.
(
issue1635)
Specifically, if:
1) the user edited the message (it didn't come straight from -m) and
2) the commit was aborted by an exception
then the saved commit message in .hg/last-message.txt could come in handy, so
mention it with a ui.write().
This doesn't help users who manually rollback to amend a changeset:
the fact that the message was saved to .hg/last-message.txt is
invisible in that case.