Teach git to read various information from $XDG_CONFIG_HOME/git/ to allow
the user to avoid cluttering $HOME.
* mm/config-xdg:
config: write to $XDG_CONFIG_HOME/git/config file when appropriate
Let core.attributesfile default to $XDG_CONFIG_HOME/git/attributes
Let core.excludesfile default to $XDG_CONFIG_HOME/git/ignore
config: read (but not write) from $XDG_CONFIG_HOME/git/config file