git.el: Don't use font-lock-compile-keywords

git.el: Don't use font-lock-compile-keywords

If font-lock is disabled, font-lock-compile-keywords complains.
Really what we want to do is to replace log-edit's font-lock
definitions with our own, so define a major mode deriving from
log-edit and set up font-lock-defaults there.  We then use the
optional MODE argument to log-edit to set up the major mode of the
commit buffer appropriately.

Signed-off-by: Lawrence Mitchell <wence@gmx.li>
Acked-by: Alexandre Julliard <julliard@winehq.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>