localrepo: add a way to get the current lock if it's held

We'll use this in upcoming patches to return a reference to the wlock if it's
held.

mercurial/localrepo.py

@@ -1300,6 +1300,15 b' class localrepository(object):'
         self._wlockref = weakref.ref(l)
         return l
 
+    def _currentlock(self, lockref):
+        """Returns the lock if it's held, or None if it's not."""
+        if lockref is None:
+            return None
+        l = lockref()
+        if l is None or not l.held:
+            return None
+        return l
+
     def _filecommit(self, fctx, manifest1, manifest2, linkrev, tr, changelist):
         """
         commit an individual file as part of a larger transaction