When the user specifies a message, use fmt_merge_msg_shortlog() to
append the shortlog.

Previously, when a message was specified, we ignored the merge title
("Merge <foo> into <bar>") and shortlog from fmt_merge_msg().

Update the documentation for -m to reflect this too.
Signed-off-by: NTay Ray Chuan <rctay89@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>