Apart from the list of "valid values", 'diff.tool' can take any value,
provided there is a corresponding 'difftool.<tool>.cmd' option. Also,
describe this option just before the 'difftool.*' options.
Helped-by: Michael J Gruber <git@drmicha.warpmail.net> Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>