config: write to $XDG_CONFIG_HOME/git/config file when appropriate

config: write to $XDG_CONFIG_HOME/git/config file when appropriate

Teach git to write to $XDG_CONFIG_HOME/git/config if

 - it already exists,
 - $HOME/.gitconfig file doesn't, and
 - The --global option is used.

Otherwise, write to $HOME/.gitconfig when the --global option is
given, as before.

If the user doesn't create $XDG_CONFIG_HOME/git/config, there is
absolutely no change. Users can use this new file only if they want.

If $XDG_CONFIG_HOME is either not set or empty, $HOME/.config/git/config
will be used.

Advice for users who often come back to an old version of Git: you
shouldn't create this file.

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>