git-sh-setup.sh: make GIT_DIR absolute

git-sh-setup.sh: make GIT_DIR absolute

Quite a few of the scripts are rather careless about using GIT_DIR
while changing directories.

Some try their hands (with different likelihood of success) in making
GIT_DIR absolute.

This patch lets git-sh-setup.sh cater for absolute directories (in a
way that should work reliably also with non-Unix path names) and
removes the respective kludges in git-filter-branch.sh and
git-instaweb.sh.

Signed-off-by: David Kastrup <dak@gnu.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>