gc.sh: partition repacking refs more robustly

gc.sh: partition repacking refs more robustly

Move the location of any potentially updated-during-repacking refs
to a completely distinct refs namespace so that there's absolutely
no possibility of a collision.

The possibility of a collision before was very remote and then if
the stars aligned the worst that could have happened was that one
or more objects that should have been placed in the pack ended up
loose instead and perhaps the project's bundle failed to be created
as a result (it might not have been complete in that case).

There are no known occurrences of this happening in the wild, but
it's better to entirely eliminate the possibility just to be safe.

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