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