mergetool-lib: make the three-way diff the default for vim/gvim
mergetool-lib: make the three-way diff the default for vim/gvim

The original vimdiff/gvimdiff configuration is now available by using
'vimdiff2' or 'gvimdiff2' as the preferred merge tool.

Signed-off-by: Dan McGee <a6e5737275ff1276377ee261739f3ee963671241@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git