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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

# So GitLab doesn't think we're using tons of Perl
*.t -linguist-detectable