revert: improve success message by adding abbreviated commit sha1

revert: improve success message by adding abbreviated commit sha1

Instead of saying "Finished one cherry-pick." or "Finished one revert.",
we now say "Finished cherry-pick of commit <abbreviated sha1>." or
"Finished revert of commit <abbreviated sha1>." which is more informative,
especially when cherry-picking or reverting many commits.

In case of failure the message is now "Automatic cherry-pick of commit
<abbreviated sha1> failed." instead of "Automatic cherry-pick failed."

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>