Fall back to three-way merge when applying a patch.

Fall back to three-way merge when applying a patch.

After git-apply fails, attempt to find a base tree that the patch
cleanly applies to, and do a three-way merge using that base tree into
the current index, if .dotest/.3way file exists.  This flag can be
controlled by giving -m flag to git-applymbox command.

When the fall-back merge fails, the working tree can be resolved the
same way as you would normally hand resolve a conflicting merge.
When making commit, use .dotest/final-commit as the log message
template.  Or you could just choose to 'git-checkout-index -f -a'
to revert the failed merge.

Signed-off-by: Junio C Hamano <junkio@cox.net>