git-p4: Teach gitConfig method about arguments.

git-p4: Teach gitConfig method about arguments.

With this patch, it is possible to call the gitConfig method with an optional
argument string, which will be passed to the "git config" executable. For
instance:

gitConfig("core.ignorecase", "--bool")

will ensure that you get the value "true", and won't have to check the returned
value for [1, true, on, yes].

Signed-off-by: Tor Arvid Lund <torarvid@gmail.com>
Acked-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>