Fix setting config variables with an alternative GIT_CONFIG
When setting a config variable, git_config_set() ignored the variables
GIT_CONFIG and GIT_CONFIG_LOCAL. Now, when GIT_CONFIG_LOCAL is set, it
will write to that file. If not, GIT_CONFIG is checked, and only as a
fallback, the change is written to $GIT_DIR/config.
Add a test for it, and also future-proof the test for the upcoming
$HOME/.gitconfig support.
Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>