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)
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