revset: use localrepo revbranchcache for branch name filtering Branch name filtering in revsets was expensive. For every rev it created a changectx and called .branch() which retrieved the branch name from the changelog. Instead, use the revbranchcache. The revbranchcache is used read-only. The revset implementation with generators and callbacks makes it hard to figure out when we are done using/updating the cache and could write it back. It would also be 'tricky' to lock the repo for writing from within a revset execution. Finally, the branchmap update will usually make sure that the cache is updated before any revset can be run. The revbranchcache is used without any locking but is short-lived and used in a tight loop where we can assume that the changelog doesn't change ... or where it not is relevant to us if it does. perfrevset 'branch(mobile)' on mozilla-central. Before: ! wall 10.989637 comb 10.970000 user 10.940000 sys 0.030000 (best of 3) After, no cache: ! wall 7.368656 comb 7.370000 user 7.360000 sys 0.010000 (best of 3) After, with cache: ! wall 0.528098 comb 0.530000 user 0.530000 sys 0.000000 (best of 18) The performance improvement even without cache come from being based on branchinfo on the changelog instead of using ctx.branch(). Some tests are added to verify that the revbranchcache works and keep an eye on when the cache files actually are updated.