home_config_paths(): let the caller ignore xdg path

home_config_paths(): let the caller ignore xdg path

The caller can signal that it is not interested in learning
the location of $HOME/.gitconfig by passing global=NULL, but
there is no way to decline the path to the configuration
file based on $XDG_CONFIG_HOME.

Allow the caller to pass xdg=NULL to signal that it is not
interested in the XDG location.

Commit-message-by: Junio C Hamano <redacted>
Signed-off-by: Matthieu Moy <redacted>
Signed-off-by: Junio C Hamano <redacted>