This commit teaches difftool to handle the '--no-gui' option. This option
negates the existing '--gui' option. The last setting given on the command
line wins.
This allows a user to configure "[alias] mdt = difftool --gui", but still
have the ability to override the setting without error:
$ git mdt --no-gui
Signed-off-by: Tim Henigan <tim.henigan@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>