branchcache: fetch source branchcache during clone (issue3378) Recomputing branch cache on clone may be expensive, therefore if possible we fetch it along with the data. - If the clone is performed by copying, we just copy branchcache file. - If we localrepo.clone and streaming then we follow the procedure: 1. Fetch branchmap from the remote 2. Fetch the actual data. 3. Find the latest rev within branch heads (tip at the time of branchmap fetch) 4. Update the cache for the revs in [remotetip+1, tip] This way we ensure that the branchcache is correct even in case of races with commits.