dirstate-v2: complain early on docket name collision

The alternative is that the dirstate gets deleted so the
corruption persists and is hard to investigate.

This happened to me in tests, where the dirstate names
are taken from file, since the file got reverted.
I expect this can also happen in prod with non-trivial
probability (1/4 billion).

mercurial/dirstatemap.py

@@ -114,6 +114,8 b' class _dirstatemapcommon:'
         new_docket = docketmod.DirstateDocket.with_new_uuid(
             self.parents(), len(packed), meta
         )
+        if old_docket.uuid == new_docket.uuid:
+            raise error.ProgrammingError(b'dirstate docket name collision')
         data_filename = new_docket.data_filename()
         self._opener.write(data_filename, packed)
         # Write the new docket after the new data file has been