perf: make sure to explicitly disable any profiler after the first iteration

The current code work, because of some edge behavior of the `profile` class. We
make it explicit that the profiler is not in effect more than once.

contrib/perf.py

@@ -275,6 +275,8 b' class noop(object):'
     def __exit__(self, *args):
         pass
 
+NOOPCTX = noop()
+
 def gettimer(ui, opts=None):
     """return a timer function and formatter: (timer, formatter)
 
@@ -405,7 +407,7 b' def _timer(fm, func, setup=None, title=N'
     begin = util.timer()
     count = 0
     if profiler is None:
-        profiler = noop()
+        profiler = NOOPCTX
     for i in xrange(prerun):
         if setup is not None:
             setup()
@@ -417,6 +419,7 b' def _timer(fm, func, setup=None, title=N'
         with profiler:
             with timeone() as item:
                 r = func()
+        profiler = NOOPCTX
         count += 1
         results.append(item[0])
         cstop = util.timer()