gc.sh: only copy down .pack and .idx pack files
When hard-linking the parents objects and packs
into the child fork, avoid extraneous pack files
(such as pack-*.bitmap). Only copy down the
loose objects and pack .idx and .pack files.
This prevents a situation where a fork can end up
seeing more than one .bitmap file (the old one that
was copied in and the new one generated in the parent).
The git client will display a warning message if
more than one .bitmap file is visible and we want
to avoid that.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>