mergetool: Add prompt to continue after failing to merge a file

mergetool: Add prompt to continue after failing to merge a file

This option stops git mergetool from aborting at the first failed merge.
After a failed merge the user will be prompted to indicated whether he
wishes to continue with attempting to merge subsequent paths or to
abort.

This allows some additional use patterns. Merge conflicts can now be
previewed one at time and merges can also be skipped so that they can be
performed in a later pass.

Signed-off-by: Charles Bailey <charles@hashpling.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>