Make prune also run prune-packed

Make prune also run prune-packed

Both the git-prune manpage and everday.txt say that git-prune should also prune
unpacked objects that are also found in packs, by running git prune-packed.

Junio thought this was "a regression when prune was rewritten as a built-in."

So modify prune to call prune-packed again.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>