Fix mdiff.py type annotations and remove pyre-fixme suppressions

[anushamukka-dev]

Summary:
mdiff.py carried 29 pyre-fixme suppressions, almost all stemming from
two root causes:

• Functions that operate on diff content were annotated as str (or the
  overly broad Sized) when they actually take bytes. This mislabeling
  forced pyre-fixme[9]/[6] at nearly every internal call site.
• diffopts populates its option fields dynamically via setattr, so it
  declared none of them; every opts.<field> access tripped
  pyre-fixme[16].

Fixes, matching how the code actually behaves at runtime:

• wsclean, splitblock, allblocks, and unidiff now take bytes
  (unidiff's a is Optional[bytes], reflecting the existing
  a is None handling for added files).
• diffopts declares its option fields as class-level attribute
  annotations, so attribute access type-checks without changing the
  setattr-based initialization.
• _unidiff's return type admits the leading bool sentinel it yields
  (Iterator[Union[bool, ...]]).
• The two Optional-subscript fixmes are resolved by reading/writing
  through the prev alias, which pyre can prove is non-None, instead of
  the Optional hunk.

All 29 suppressions are removed with no runtime behavior change (the
splitblock and hunk/prev edits are behavior-preserving renames/alias
uses).

Note: diffopts is a concrete class here, so class-level attribute
annotations are used rather than a separate Protocol — this is the minimal
change that makes every opts.<field> access resolve.

Differential Revision: D112392597

[meta-codesync]

@anushamukka-dev has exported this pull request. If you are a Meta employee, you can view the originating Diff in D112392597.