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>