git-gc: by default use safer "-A" option to repack when not --prune'ing

git-gc: by default use safer "-A" option to repack when not --prune'ing

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: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>