typing: add missing signature for mercurial.cext.parsers.parse_index2()

typing: add missing signature for mercurial.cext.parsers.parse_index2()

Flagged by pytype 2022.11.18 when the cext stubs are made visible to it.  There
are very likely other signatures that are missing, but this is enough to keep it
happy for now.

--- a/mercurial/cext/parsers.pyi	Wed Nov 23 11:22:22 2022 -0500
+++ b/mercurial/cext/parsers.pyi	Wed Nov 23 15:50:20 2022 -0500
@@ -76,3 +76,7 @@
 
     def insert(self, rev: int) -> None: ...
     def shortest(self, node: bytes) -> int: ...
+
+# The IndexObject type here is defined in C, and there's no type for a buffer
+# return, as of py3.11.  https://github.com/python/typing/issues/593
+def parse_index2(data: object, inline: object, format: int = ...) -> Tuple[object, Optional[Tuple[int, object]]]: ...