Add sub-command to check if maintenance is needed

The git-pack-refs(1) command repacks references. It supports an --auto flag which uses heuristics to decide and perform repacking.

Gitaly uses an expensive write transaction to perform repacking. However, it would be nice if Git exposed an option to check if the repository even needs repacking so the top level read transaction can decide if the creating the write transaction is even needed.

This will be added as an extension to git maintenance, perhaps something like git maintenance needed which would imply if maintenance as a whole on the repository is needed or not (refs and objects).