git-hooks/zephyr-post-receive: Portability improvements

git-hooks/zephyr-post-receive: Portability improvements

$() command substitution is a bashism, so use bash instead of sh
in the shebang line.

'readlink -f' isn't available everywhere, and we're not actually
worried about canonicalizing the path so much as avoiding an
uninformative "." as the name of the repo.  Just cd into the
directory and look at pwd.

Signed-off-by: Mitchell E Berger <mitchb@mit.edu>