branchmap: use mmap for faster revbranchcache loading
A typical revbranchmap usage is:
- load the entire revbranchmap file into memory
- maybe do a few lookups
- add a few bytes to it
- write the addition to disk
There's no reason to load the entire revbranchmap into memory.
We can split it into a large immutable prefix and a mutable suffix,
and then memorymap the prefix, thus saving all the useless loading.
Benchmarking on some real-world pushes suggests that out of ~100s server-side
push handling revbranchcache handling is responsible for:
* ~7s with no change
* ~1.3s with the change, without mmap
* 0.04s with the change, with mmap
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}