We already have xdg_config_home to format paths relative to
XDG_CONFIG_HOME. Let's provide a similar function xdg_cache_home to do
the same for paths relative to XDG_CACHE_HOME.
Signed-off-by: Devin Lehmacher <lehmacdj@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>