lock: turn a lock into a Python context manager

lock: turn a lock into a Python context manager

This lets us greatly simply acquire/release cycles.

Code pattern before:

  try:
    lock = repo.lock()
    # zillions of lines of code
  finally:
    lock.release()

And after:

  with repo.lock():
    # ...

--- a/mercurial/lock.py	Fri Jan 15 13:14:50 2016 -0800
+++ b/mercurial/lock.py	Fri Jan 15 13:14:45 2016 -0800
@@ -58,6 +58,12 @@
         if self.acquirefn:
             self.acquirefn()
 
+    def __enter__(self):
+        return self
+
+    def __exit__(self, exc_type, exc_value, exc_tb):
+        self.release()
+
     def __del__(self):
         if self.held:
             warnings.warn("use lock.release instead of del lock",