git-sh-setup: Use "cd" option, not /bin/pwd, for symlinked work tree

git-sh-setup: Use "cd" option, not /bin/pwd, for symlinked work tree

In cd_to_toplevel, instead of 'cd $(unset PWD; /bin/pwd)/$path'
use 'cd -P $path'.  The "-P" option yields a desirable similarity to
C chdir.

While the "-P" option may be slightly less commonly supported than
/bin/pwd, it is more concise, better tested, and less error prone.
I've already added the 'unset PWD' to fix the /bin/pwd solution on
BSD; there may be more edge cases out there.

This still passes all the same test cases in t5521-pull-symlink.sh and
t2300-cd-to-toplevel.sh, even before updating them to use 'pwd -P'.

Signed-off-by: Junio C Hamano <gitster@pobox.com>