Added example hook script to save/restore permissions/ownership.

Added example hook script to save/restore permissions/ownership.

Usage info is emebed in the script, but the gist of it is to run the script
from a pre-commit hook to save permissions/ownership data to a file and check
that file into the repository.  Then, a post_merge hook reads the file and
updates working tree permissions/ownership.  All updates are transparent to
the user (although there is a --verbose option).  Merge conflicts are handled
in the "read" phase (in pre-commit), and the script aborts the commit and
tells you how to fix things in the case of a merge conflict in the metadata
file.  This same idea could be extended to handle file ACLs or other file
metadata if desired.

Signed-off-by: Josh England <jjengla@sandia.gov>
Signed-off-by: Junio C Hamano <gitster@pobox.com>