repo-config: support --get-regexp

With --get-regexp, output all key/value pairs where the key matches a
regexp. Example:

	git-repo-config --get-regexp remote.*.url

will output something like

	remote.junio.url git://git.kernel.org/pub/scm/git/git.git
	remote.gitk.url git://git.kernel.org/pub/scm/gitk/gitk.git

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>