fsmonitor: do not nuke dirstate filecache

In the future, chg may prefill repo's dirstate filecache so it's valuable
and should be kept. Previously we drop both filecache and property cache for
dirstate during fsmonitor reposetup, this patch changes it to only drop
property cache but keep the filecache.

hgext/fsmonitor/__init__.py

@@ -692,11 +692,13 b' def reposetup(ui, repo):'
 
         # at this point since fsmonitorstate wasn't present, repo.dirstate is
         # not a fsmonitordirstate
-        repo.dirstate.__class__ = makedirstate(repo.dirstate.__class__)
-        # nuke the dirstate so that _fsmonitorinit and subsequent configuration
-        # changes take effect on it
-        del repo._filecache['dirstate']
-        delattr(repo.unfiltered(), 'dirstate')
+        dirstate = repo.dirstate
+        dirstate.__class__ = makedirstate(dirstate.__class__)
+        dirstate._fsmonitorinit(fsmonitorstate, client)
+        # invalidate property cache, but keep filecache which contains the
+        # wrapped dirstate object
+        del repo.unfiltered().__dict__['dirstate']
+        assert dirstate is repo._filecache['dirstate'].obj
 
         class fsmonitorrepo(repo.__class__):
             def status(self, *args, **kwargs):