localrepo: do not cache auditor/nofsauditor which would make reference cycle

Before, self.auditor and self.nofsauditor held self through self._checknested,
and the following code couldn't free a repo by ref-counting.

  def main():
      repo = localrepo.localrepository(uimod.ui(), '../testrepos/hello')
  main()

With this change, the cache of the nofsauditor is limited to a single match
session. I think that's okay as the nofsauditor doesn't do any filesystem
access. Alternatively, maybe we can remove the callback from nofsauditor
since it isn't used unless realfs=True, but I have no idea whether it is
a bug or not.

mercurial/localrepo.py

@@ -436,14 +436,6 b' class localrepository(object):'
         self.root = self.wvfs.base
         self.path = self.wvfs.join(".hg")
         self.origroot = path
-        # This is only used by context.workingctx.match in order to
-        # detect files in subrepos.
-        self.auditor = pathutil.pathauditor(
-            self.root, callback=self._checknested)
-        # This is only used by context.basectx.match in order to detect
-        # files in subrepos.
-        self.nofsauditor = pathutil.pathauditor(
-            self.root, callback=self._checknested, realfs=False, cached=True)
         self.baseui = baseui
         self.ui = baseui.copy()
         self.ui.copy = baseui.copy # prevent copying repo configuration
@@ -709,6 +701,22 b' class localrepository(object):'
     def _writerequirements(self):
         scmutil.writerequires(self.vfs, self.requirements)
 
+    # Don't cache auditor/nofsauditor, or you'll end up with reference cycle:
+    # self -> auditor -> self._checknested -> self
+
+    @property
+    def auditor(self):
+        # This is only used by context.workingctx.match in order to
+        # detect files in subrepos.
+        return pathutil.pathauditor(self.root, callback=self._checknested)
+
+    @property
+    def nofsauditor(self):
+        # This is only used by context.basectx.match in order to detect
+        # files in subrepos.
+        return pathutil.pathauditor(self.root, callback=self._checknested,
+                                    realfs=False, cached=True)
+
     def _checknested(self, path):
         """Determine if path is a legal nested repository."""
         if not path.startswith(self.root):