dirstate-tree: Avoid BTreeMap double-lookup when inserting a dirstate entry The child nodes of a given node in the tree-shaped dirstate are kept in a `BTreeMap` where keys are file names as strings. Finding or inserting a value in the map takes `O(log(n))` string comparisons, which adds up when constructing the tree. The `entry` API allows finding a "spot" in the map that may or may not be occupied and then access that value or insert a new one without doing map lookup again. However the current API is limited in that calling `entry` requires an owned key (and so a memory allocation), even if it ends up not being used in the case where the map already has a value with an equal key. This is still a win, with 4% better end-to-end time for `hg status` measured here with hyperfine: ``` Benchmark #1: ../hg2/hg status -R $REPO --config=experimental.dirstate-tree.in-memory=1 Time (mean ± σ): 1.337 s ± 0.018 s [User: 892.9 ms, System: 437.5 ms] Range (min … max): 1.316 s … 1.373 s 10 runs Benchmark #2: ./hg status -R $REPO --config=experimental.dirstate-tree.in-memory=1 Time (mean ± σ): 1.291 s ± 0.008 s [User: 853.4 ms, System: 431.1 ms] Range (min … max): 1.283 s … 1.309 s 10 runs Summary './hg status -R $REPO --config=experimental.dirstate-tree.in-memory=1' ran 1.04 ± 0.02 times faster than '../hg2/hg status -R $REPO --config=experimental.dirstate-tree.in-memory=1' ``` * ./hg is this revision * ../hg2/hg is its parent * $REPO is an old snapshot of mozilla-central Differential Revision: https://phab.mercurial-scm.org/D10550