lock: add a method to reacquire the lock after subprocesses exit

This is part of a series that will allow locks to be inherited by subprocesses
in limited circumstances.

mercurial/lock.py

@@ -180,6 +180,14 b' class lock(object):'
         self._inherited = True
         return lockname
 
+    def reacquire(self):
+        if not self._inherited:
+            raise error.LockInheritanceContractViolation(
+                'reacquire can only be called after prepinherit')
+        if self.acquirefn:
+            self.acquirefn()
+        self._inherited = False
+
     def release(self):
         """release the lock and execute callback function if any