builtin/config: pull out function to handle config location

builtin/config: pull out function to handle config location

There's quite a bunch of options to git-config(1) that allow the user to
specify which config location to use when reading or writing config
options. The logic to handle this is thus by necessity also quite
involved.

Pull it out into a separate function so that we can reuse it in
subsequent commits which introduce proper subcommands.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>