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).

typing: fix a syntax error in mercurial/cext/bdiff.pyi I noticed because `merge-pyi` on the non-cext implementations with this file as input skipped the return type for this. `pytype-single --parse-pyi` confirmed it was a problem.