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.
--- a/mercurial/cext/bdiff.pyi Fri Nov 04 19:38:47 2022 -0400
+++ b/mercurial/cext/bdiff.pyi Tue Nov 08 13:38:06 2022 -0500
@@ -5,7 +5,7 @@
version: int
-def bdiff(a: bytes, b: bytes): bytes
+def bdiff(a: bytes, b: bytes) -> bytes: ...
def blocks(a: bytes, b: bytes) -> List[Tuple[int, int, int, int]]: ...
def fixws(s: bytes, allws: bool) -> bytes: ...
def splitnewlines(text: bytes) -> List[bytes]: ...