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).

mercurial/cffi/bdiff.py

@@ -29,7 +29,7 b' def blocks(sa, sb):'
         count = lib.bdiff_diff(a[0], an, b[0], bn, l)
         if count < 0:
             raise MemoryError
-        rl = [None] * count
+        rl = [(0, 0, 0, 0)] * count
         h = l.next
         i = 0
         while h: