typing: add several assertions to dirstatemap to appease pytype stable

typing: add several assertions to dirstatemap to appease pytype

(grafted from default to stable)

I think it's been mentioned in IRC that these can't be None in this case.  This
fixes:

    File "/mnt/c/Users/Matt/hg/mercurial/dirstatemap.py", line 213, in addfile: unsupported operand type(s) for &: 'None' and 'int' [unsupported-operands]
      No attribute '__and__' on None or '__rand__' on int
    Called from (traceback):
      line 290, in reset_state
    File "/mnt/c/Users/Matt/hg/mercurial/dirstatemap.py", line 214, in addfile: unsupported operand type(s) for &: 'None' and 'int' [unsupported-operands]
      No attribute '__and__' on None or '__rand__' on int
    Called from (traceback):
      line 290, in reset_state

Differential Revision: https://phab.mercurial-scm.org/D11235

--- a/mercurial/dirstatemap.py	Mon Aug 02 10:51:19 2021 -0400
+++ b/mercurial/dirstatemap.py	Fri Jul 30 00:11:56 2021 -0400
@@ -209,6 +209,9 @@
         else:
             assert size != FROM_P2
             assert size != NONNORMAL
+            assert size is not None
+            assert mtime is not None
+
             state = b'n'
             size = size & rangemask
             mtime = mtime & rangemask