jobd/gc.sh: compute newdeltas setting after make_repack_dir

jobd/gc.sh: compute newdeltas setting after make_repack_dir

In order to prepare for checking the "repack" "signature" hash,
move the computation of the `$newdeltas` setting to after the call
to `make_repack_dir`.

This is necessary because during the computation of the `$newdeltas`
setting, in certain circumstances, "mini" maintenance gets performed.

That "mini" maintenance could potentially disturb the current
set of packs in the repository.

However, if we are potentially in a `$skipgc` situation and the
final "repack" "signature" hash matches, then we will abort gc
early with nothing to do and we need to be able to do that before
any "mini" maintenance takes place while at the same time the
check cannot be done until after `make_repack_dir` has been run.

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