treemanifest: lazily load manifests Most operations on treemanifests already visit only relevant submanifests. Notable examples include __getitem__, __contains__, walk/matches with matcher, diff. By making submanifests lazily loaded, we speed up all these operations. The lazy loading is achieved by adding a _load() method that gets defined where we currently eagerly parse the manifest. We make sure to call it before any access to _dirs, _files or _flags. Some timings on the Mozilla repo (with flat manifest timings for reference): hg cat -r . README.txt: 1.644s -> 0.096s (0.255s) hg diff -r .^ -r . : 1.746s -> 0.137s (0.431s) hg files -r . python : 1.508s -> 0.146s (0.335s) hg files -r . : 2.125s -> 2.203s (0.712s)