gc.sh: do not skip "mini" maintenance when forks are present
Normally, if all deltas are being recomputed during a full gc,
the "mini" maintenance activities are a waste of time because
doing a full gc and recomputing all deltas would duplicate effort.
However, in the presence of forks, it's important to perform
these activities before copying stuff down to the forks to avoid
suboptimal packs living on in the forks or excessive numbers of
loose objects being copied down to the forks.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>