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