The fast-forward logic is never being triggered because $common and
$MRC are never equivalent. $common is initialized to a commit id by
merge-base and MRC is initialized to HEAD. Fix this by initializing
$MRC to the commit id for HEAD so that its possible for $MRC and
$common to be equal.
Signed-off-by: Stephen Boyd <bebarino@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>