match: improve includematcher.visitchildrenset to be much faster and cached This improves the speed of visitchildrenset considerably, especially when there are complicated matchers involved that may have many entries in _dirs or _parents. Unfortunately the benchmark isn't easily upstreamed due to its reliance on https://github.com/vstinner/perf (primarily due to the conflict when importing it if I were to contribute the benchmark as contrib/matcherbenchmarks.py) instead of asv or some other perf measurement system. To describe the benchmark briefly: I generated an includematcher of either 5 or 3500 "rootfilesin:prefix1/prefix2/prefix3/<randomsubdirs, 1-8 levels deep>" items in the 'setup' function, and then called `im.visitchildrenset('prefix1/prefix2')` in the 'stmt' function in perf.timeit. For the set of 5: - before: 15.3 us +- 2.9 us - after: 1.59 us +- 0.02 us For the set of 3500: - before: 3.90 ms +- 0.10 ms - after: 3.15 us +- 0.09 us (note the m->u change) Differential Revision: https://phab.mercurial-scm.org/D4351