Add $GIT_CONFIG environment variable whose content is used instead
of .git/config if set. Also add $GIT_CONFIG_LOCAL as a
forward-compatibility cue for whenever we will finally come to support]
global configuration files (properly).
Signed-off-by: Petr Baudis <pasky@suse.cz> Signed-off-by: Junio C Hamano <junkio@cox.net>