pre-receive: tolerate an unlimited number of .pack files

pre-receive: tolerate an unlimited number of .pack files

The pre-receive hook attempts to make sure any just-received
packs are writable by user and group so they can be successfully
touched later during gc.

Previously the expression objects/pack/pack-$octet20.pack was used
to pass a list of pack filenames to chmod.  However, in the
presence of an obscene number of .pack files, that expression could
result in an expansion that exceeds the command line length
limitation.

Replace the problematic line with a find | xargs chmod pipe so that
any number of .pack files can be accomodated (there really SHOULD
never be all that many).  This also allows an optimization since find
can exclude .pack files that already have ug+w permissions.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>