transaction: turn a transaction into a Python context manager

This lets us greatly simply acquire/release cycles.

If the block completes without raising an exception, the transaction
is closed.

Code pattern before:

  try:
    tr = repo.transaction('x')
    # zillions of lines of code
    tr.close()
  finally:
    tr.release()

And after:

  with tr.transaction('x'):
    # ...

mercurial/transaction.py

@@ -333,6 +333,14 b' class transaction(object):'
         if self.count > 0 and self.usages == 0:
             self._abort()
 
+    def __enter__(self):
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        if exc_type is None:
+            self.close()
+        self.release()
+
     def running(self):
         return self.count > 0