mergetools/vimdiff: trust Vim's exit code

mergetools/vimdiff: trust Vim's exit code

Allow vimdiff users to signal that they do not want to use the
result of a merge by exiting with ":cquit", which tells Vim to
exit with an error code.

This is better than the current behavior because it allows users
to directly flag that the merge is bad, using a standard Vim
feature, rather than relying on a timestamp heuristic that is
unforgiving to users that save in-progress merge files.

The original behavior can be restored by configuring
mergetool.vimdiff.trustExitCode to false.

Reported-by: Dun Peal <dunpealer@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>