git-gc: always use -A when manually repacking

git-gc: always use -A when manually repacking

Now that repack -A will leave unreferenced objects unpacked, there is
no reason to use the -a option to repack (which will discard unreferenced
objects). The unpacked unreferenced objects will not be repacked by a
subsequent repack, and will eventually be pruned by git-gc based on the
gc.pruneExpire config option.