An undocumented mis-feature in git-difftool is that it allows you
to specify a default difftool by setting GIT_MERGE_TOOL.
This behavior was never documented and was included as an
oversight back when git-difftool was maintained outside of git.
git-mergetool never honored GIT_MERGE_TOOL so neither should
git-difftool.
Signed-off-by: David Aguilar <davvid@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>