merge: remove direct rustmod reference

We shouldn't rely on this member being present in `dirstate.py`, this creates
unnecessary coupling.
This also can trigger certain issues in edge-cases where the policy is changed
at runtime or multiple Python environments fight, which is an added bonus.

Differential Revision: https://phab.mercurial-scm.org/D12217

mercurial/merge.py

@@ -25,6 +25,7 b' from . import ('
     mergestate as mergestatemod,
     obsutil,
     pathutil,
+    policy,
     pycompat,
     scmutil,
     subrepoutil,
@@ -1764,9 +1765,9 b' def _advertisefsmonitor(repo, num_gets, '
         b'fsmonitor', b'warn_update_file_count'
     )
     # avoid cycle dirstate -> sparse -> merge -> dirstate
-    from . import dirstate
+    dirstate_rustmod = policy.importrust("dirstate")
 
-    if dirstate.rustmod is not None:
+    if dirstate_rustmod is not None:
         # When using rust status, fsmonitor becomes necessary at higher sizes
         fsmonitorthreshold = repo.ui.configint(
             b'fsmonitor',