config: read (but not write) from $XDG_CONFIG_HOME/git/config file

config: read (but not write) from $XDG_CONFIG_HOME/git/config file

Teach git to read the "gitconfig" information from a new location,
$XDG_CONFIG_HOME/git/config; this allows the user to avoid
cluttering $HOME with many per-application configuration files.

In the order of reading, this file comes between the global
configuration file (typically $HOME/.gitconfig) and the system wide
configuration file (typically /etc/gitconfig).

We do not write to this new location (yet).

If $XDG_CONFIG_HOME is either not set or empty, $HOME/.config/git/config
will be used. This is in line with XDG specification.

If the new file does not exist, the behavior is unchanged.

Signed-off-by: Huynh Khoi Nguyen Nguyen <Huynh-Khoi-Nguyen.Nguyen@ensimag.imag.fr>
Signed-off-by: Valentin Duperray <Valentin.Duperray@ensimag.imag.fr>
Signed-off-by: Franck Jonas <Franck.Jonas@ensimag.imag.fr>
Signed-off-by: Lucien Kong <Lucien.Kong@ensimag.imag.fr>
Signed-off-by: Thomas Nguy <Thomas.Nguy@ensimag.imag.fr>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>