treemanifest: make treemanifest.matches() faster By converting treemanifest.matches() into a recursively additivie operation, it becomes O(n). The old matches function made a copy of the entire manifest and deleted files that didn't match. With tree manifests, this was an O(n log n) operation because del() was O(log n). This change speeds up the command "hg status --rev .^ 'relglob:*.js' on the Mozilla repo, now taking 2.53s, down from 3.51s.

treemanifest: add treemanifest._isempty() During operations that involve building up a new manifest tree, it will be useful to be able to quickly check if a submanifest is empty, and if so, to avoid including it in the final tree. Doing this check lets us avoid creating treemanifest structures that contain any empty submanifests.