Support XDG_CONFIG_HOME for ignore/config mode.

From git-config(1)

    $XDG_CONFIG_HOME/git/config
       Second user-specific configuration file. If $XDG_CONFIG_HOME is not set or empty,
       $HOME/.config/git/config will be used. Any single-valued variable set in this file
       will be overwritten by whatever is in ~/.gitconfig. It is a good idea not to
       create this file if you sometimes use older versions of Git, as support for this
       file was added fairly recently.

And gitignore(5)

    [...] Its default value is
    $XDG_CONFIG_HOME/git/ignore. If $XDG_CONFIG_HOME is either not set or empty,
    $HOME/.config/git/ignore is used instead.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Merge pull request #36 from ruediger/xdg-config

Support XDG_CONFIG_HOME for ignore/config mode.