format-patch: Don't go over merge commits

format-patch: Don't go over merge commits

If the topmost three commits in a branch were merge commits, 'git
format-patch -3' used to output nothing. Since Git can't prepare
patches out of merge commits anyway, don't go over them in the first
place. 'git format-patch -3' now prepares three patches from the
topmost three commits without counting merge commits. Also add a
corresponding test in t4014-format-patch and update documentation.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>