crecord: update uiheader docstring There's no need to move anything to patch.py. The uiheader class only has methods relevant to crecord and overrides __getattr__ in order to use `patch.header` objects as a sort of mixin.

crecord: add skipfolded param to previtem This just simplifies the API a bit so it matches `nextitem` and I can handle both nextitem and previtem symmetrically.

dispatch: don't attempt to import debugger as bytestring The __import__ thingie needs a string, not a bytestring. Guess I'm the only one who uses this once in a while and noticed it was broken.

debugsetparents: fix Marmoutian docstring Just some light proofreading.

docs: fix broken `make` in `docs/` We had some wrapped lines without blank lines between, which made the runrst script think the list was not a list and it got confused about the indentation. I added blank lines, and also some other minor styling for consistency with the rest of the file.

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