This makes use of repack's new "-A" option which does not drop packed
unreachable objects. This makes git-gc safe to call at any time,
particularly when a repository is referenced as an alternate by
another repository.

git-gc --prune will use the "-a" option to repack instead of "-A", so
that packed unreachable objects will be removed.
Signed-off-by: NBrandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: NLars Hjemli <hjemli@gmail.com>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>