Allow multiple -m options to git-commit.

Allow multiple -m options to git-commit.

I find it very convenient to be able to supply multiple paragraphs
of text on the command line with a single git-commit call.  This
change permits multiple -m/--message type options to be supplied
to git-commit with each message being added as its own paragraph
of text in the commit message.

The -m option is still not permitted with -c/-C/-F nor are multiple
occurrences of these options permitted.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>