fmt-merge-msg -m to override merge title

fmt-merge-msg -m to override merge title

Since v1.7.1.1~23^2 (merge: --log appends shortlog to message if
specified, 2010-05-11), the fmt-merge-msg backend supports custom text
to override the merge title "Merge <foo> into <bar>".

Expose this functionality for scripted callers.  Example:

 git fmt-merge-msg --log -m \
	"$(printf '%s\n' \
	    "Merge branch 'api-cleanup' into feature" \
	    '' \
	    'This is to use a few functions refactored for this purpose.'
	)" <.git/FETCH_HEAD

Cc: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>