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.