merge: do not add standard message when message is given with -m option

merge: do not add standard message when message is given with -m option

Even if the user explicitly gave her own message to "git merge", the
command still added its standard merge message.  It resulted in a
useless repetition like this:

    % git merge -m "Merge early part of side branch" `git rev-parse side~2`
    % git show -s
    commit 37217141e7519629353738d5e4e677a15096206f
    Merge: e68e646 a1d2374
    Author: しらいし ななこ <nanako3@lavabit.com>
    Date:   Wed Dec 2 14:33:20 2009 +0900

	Merge early part of side branch

	Merge commit 'a1d2374f8f52f4e8a53171601a920b538a6cec23'

The gave her own message because she didn't want git to add the
standard message (if she wanted to, she wouldn't have given one,
or she would have prepared it using git-fmt-merge-msg command).

Noticed by Nanako Shiraishi

Signed-off-by: Junio C Hamano <gitster@pobox.com>