gitk: Comply with XDG base directory specification
Write the gitk config data to $XDG_CONFIG_HOME/git/gitk ($HOME/.config/git/gitk
by default) in line with the XDG specification. This makes it consistent with
git which also follows the spec.
If $HOME/.gitk already exists use that for backward compatibility, so only new
installations are affected.
Signed-off-by: Astril Hayato <astrilhayato@gmail.com> Signed-off-by: Paul Mackerras <paulus@samba.org>