disable GC on submodules

The strategy used by git-meta, which creates no branches in submodules, leaves git-meta-created commits at risk of GC whenever the users change HEAD (e.g., `git meta checkout`). That is, commits that are logically reachable from the meta-repo are not reachable in any way visible to Git.

A couple of alternatives are, e.g.:
- create synthetic-meta-refs in submodules when you do git-meta-commit -- this is insufficient because it won't help if users create the commits in submodules with plain Git, and we certainly do not want to require users to create commits with git-meta
- create synthetic meta-refs when users change HEAD in the meta-repo -- again, we don't want to require that users use `git-meta` to change HEAD.

The downside to this strategy is that it does require users open submodules with `git meta open`. I think that this is more acceptable.