[PATCH] gitk: Use GIT_DIR where appropriate.

Some places assumed .git is the GIT_DIR, resulting heads and
tags not showing when it was run like "GIT_DIR=. gitk --all".
This is not a contrived example --- I rely on it to verify
my private copy of git.git repository before pushing it out.

Define a single procedure "gitdir" and use it.

Signed-off-by: Junio C Hamano <junkio@cox.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>