commit: give a hint when a commit message has been abandoned

If we launch an editor for the user to create a commit
message, they may put significant work into doing so.
Typically we try to check common mistakes that could cause
the commit to fail early, so that we die before the user
goes to the trouble.

We may still experience some errors afterwards, though; in
this case, the user is given no hint that their commit
message has been saved. Let's tell them where it is.

Signed-off-by: Jeff King <peff@peff.net>