rebase: restore help for rebase w/o args (issue5059) Restoring documentation accidentally removed in a9a047878e14.

shelve: use cg3 for treemanifests Similar to previous change, this teaches shelve to pick the right changegroup version for repos that use treemanifests.

repair: use cg3 for treemanifests The newly created helper changegroup.safeversion() knows to pick version 03 if the repo uses treemanifests, so just using that means we pick the right changegroup version.

changegroup: introduce safeversion() In a few places (at least repair.py and shelve.py), we want to find the best changegroup version that we can assume users of the repo will understand. For example, we choose version 01 by default, but if it's a generaldelta repo, we expect clients to support version 02 anyway, so we choose that for new bundles (for e.g. "hg strip"). Let's create a helper for this functionality in changegroup, so we can reuse it elsewhere later.