update: add a Rust fast-path when updating from null (and clean) This case is easy to detect and we have all we need to generate a valid working copy and dirstate entirely in Rust, which speeds things up considerably: On my machine updating a repo of ~300k files goes from 10.00s down to 4.2s, all while consuming 50% less system time, with all caches hot. Something to note is that further improvements will probably happen with the upcoming `InnerRevlog` series that does smarter mmap hanlding, especially for filelogs. Here are benchmark numbers on a machine with only 4 cores (and no SMT enabled) ``` ### data-env-vars.name = heptapod-public-2024-03-25-ds2-pnm # benchmark.name = hg.command.update # bin-env-vars.hg.py-re2-module = default # bin-env-vars.hg.changeset.node = <this change> # benchmark.variants.atomic-update = no # benchmark.variants.scenario = null-to-tip # benchmark.variants.worker = default default: 5.328762 ~~~~~ rust: 1.308654 (-75.44%, -4.02) ### data-env-vars.name = mercurial-devel-2024-03-22-ds2-pnm # benchmark.name = hg.command.update # bin-env-vars.hg.py-re2-module = default # bin-env-vars.hg.changeset.node = <this change> # benchmark.variants.atomic-update = no # benchmark.variants.scenario = null-to-tip # benchmark.variants.worker = default default: 1.693271 ~~~~~ rust: 1.151053 (-32.02%, -0.54) ### data-env-vars.name = mozilla-unified-2024-03-22-ds2-pnm # benchmark.name = hg.command.update # bin-env-vars.hg.py-re2-module = default # bin-env-vars.hg.changeset.node = <this change> # benchmark.variants.atomic-update = no # benchmark.variants.scenario = null-to-tip # benchmark.variants.worker = default default: 38.901613 ~~~~~ rust: 11.637880 (-70.08%, -27.26) ### data-env-vars.name = netbsd-xsrc-public-2024-09-19-ds2-pnm # benchmark.name = hg.command.update # bin-env-vars.hg.py-re2-module = default # bin-env-vars.hg.changeset.node = <this change> # benchmark.variants.atomic-update = no # benchmark.variants.scenario = null-to-tip # benchmark.variants.worker = default default: 4.793727 ~~~~~ rust: 1.505905 (-68.59%, -3.29) ```