debug: add an option to display statistic about a bundling operation This will helps a lot to understand how the bundling decision might impact pull/unbundle on the other side.

delta-find: add debug information about reuse of cached data This will help us to understand the behavior of find-delta during a pull.

cffi: fix a bytes vs str issue on macOS when listing directories This code hasn't been touched in recent years, and the other implementation return bytes for the filename, so I assume this is a holdover from the py2 days. I was unable to test it on mac though, because the `_osutil` import failed.

typing: fix the typehint for `skip` arg on `osutil.listdir()` to be optional The cffi and pure implementations both default to `None`, and the args are parsed in C with `"y#|OO:listdir"`, with the C variable initialized to NULL.

typing: add type hints to mpatch implementations Again, using `merge-pyi` to apply the stubs in cext and then manually type the private methods. The generated stub without these hints inferred very little, and the stuff it did was wrong.

typing: add type hints to bdiff implementations Not super important code, but this was an exercise in using `merge-pyi` to fold type stubs back into the code on something small. The cext stubs don't seem to be getting used (at least the only thing in `.pytype/pyi/mercurial/cext` after a run generating the stubs is `__init__.pyi`), so maybe this will help some.

cffi: adjust the list returned by bdiff.blocks to never have a None entry This was flagged by pytype after merging the corresponding bdiff.pyi in cext: File ".../mercurial/cffi/bdiff.py", line 44, in blocks: bad return type [bad-return-type] Expected: List[Tuple[int, int, int, int]] Actually returned: List[None] AFAICT, all callers immediately unpack the tuple into 4 variables, so a `None` entry would simply crash if they aren't all overwritten. As long a `count` and the link list are consistent, this shouldn't be a problem. This placates both pytype and PyCharm (which complained about the `i` in `rl[i]` having the wrong type with the old code).