filter-branch -d: Export GIT_DIR earlier

filter-branch -d: Export GIT_DIR earlier

The improved error handling catches a bug in filter-branch when using
-d pointing to a path outside any git repository:

$ git filter-branch -d /tmp/foo master
fatal: Not a git repository (or any of the parent directories): .git

This error message comes from git for-each-ref in line 224. GIT_DIR is
set correctly by git-sh-setup (to the foo.git repository), but not
exported (yet).

Signed-off-by: Lars Noschinski <lars@public.noschinski.de>
Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>