config: make git_config_parse_parameter a public function

config: make git_config_parse_parameter a public function

We use this internally to parse "git -c core.foo=bar", but
the general format of "key=value" is useful for other
places.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>