Add --add option to git-repo-config

Add --add option to git-repo-config

For multivars, the "git-repo-config name value ^$" is useful but
nonintuitive and troublesome to do repeatedly (since the value is not
at the end of the command line).  This commit simply adds an --add
option that adds a new value to a multivar.  Particularly useful for
tracking a new branch on a remote:

git-repo-config --add remote.origin.fetch +next:origin/next

Includes documentation and test.

Signed-off-by: Brian Gernhardt <benji@silverinsanity.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>