git-sh-setup: Fix scripts whose PWD is a symlink to a work-dir on OS X

git-sh-setup: Fix scripts whose PWD is a symlink to a work-dir on OS X

On Mac OS X and possibly BSDs, /bin/pwd reads PWD from the environment if
available and shows the logical path by default rather than the physical
one.

Unset PWD before running /bin/pwd in both cd_to_toplevel and its test.

Still use the external /bin/pwd because in my Bash on Linux, the builtin
pwd prints the same result whether or not PWD is set.

Signed-off-by: Marcel M. Cary <marcel@oak.homeunix.org>
Tested-by: Wincent Colaiuta <win@wincent.com> (on Mac OS X 10.5.5)
Tested-by: Marcel Koeppen <git-dev@marzelpan.de> (on Mac OS X 10.5.6)
Signed-off-by: Junio C Hamano <gitster@pobox.com>