user-manual: fix the description of fast-forward The "Fast-forward merges" section of user-manual.txt incorrectly says if the current branch is a descendant of the other, Git will perform a fast-forward merge, but it should the other way around. Signed-off-by: Xue Fuqiao <xfq.free@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>