If config values are queried using 'git config' (e.g. via --get,
--get-all, --get-regexp, or --list flag) then it is sometimes hard to
find the configuration file where the values were defined.

Teach 'git config' the '--show-origin' option to print the source
configuration file for every printed value.
Based-on-patch-by: NJeff King <peff@peff.net>
Signed-off-by: NLars Schneider <larsxschneider@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>