typing: explicitly set the return type of `_InnerRevLog.raw_text()`

Somewhere between cd72a88c5599 and 2fd44b3dcc33, pytype changed the return type
from `Tuple[_T1, Any, bool]` to `Any`.  Both are wrong.  `mdiff.patches()` is an
alias for `mpatch.patches()`, which is selected via module policy (and breaks
the ability to infer the types).  However, `cext`, `cffi`, and `pure`
implementations all agree it returns bytes.

mercurial/revlog.py

@@ -1019,7 +1019,7 class _InnerRevlog:
         chunks.sort()
         return [x[1] for x in chunks]
 
-    def raw_text(self, node, rev):
+    def raw_text(self, node, rev) -> bytes:
         """return the possibly unvalidated rawtext for a revision
 
         returns rawtext