profile: introduce a knob to control if the context is actually profiling

profile: introduce a knob to control if the context is actually profiling

This is a step toward allowing context where the profiling in enabled
withing the context range.

This also open the way to kill the dedicated "maybeprofile" context manager
and keep only one of 'profile' and 'maybeprofile'.

--- a/mercurial/profiling.py	Fri Jun 09 11:39:53 2017 +0100
+++ b/mercurial/profiling.py	Fri Jun 09 11:41:47 2017 +0100
@@ -147,17 +147,19 @@
     Profiling is active when the context manager is active. When the context
     manager exits, profiling results will be written to the configured output.
     """
-    def __init__(self, ui):
+    def __init__(self, ui, enabled=True):
         self._ui = ui
         self._output = None
         self._fp = None
         self._profiler = None
+        self._enabled = enabled
         self._entered = False
         self._started = False
 
     def __enter__(self):
         self._entered = True
-        self.start()
+        if self._enabled:
+            self.start()
 
     def start(self):
         """Start profiling.
@@ -228,8 +230,5 @@
     just use a single code path for calling into code you may want to profile
     and this function determines whether to start profiling.
     """
-    if ui.configbool('profiling', 'enabled'):
-        with profile(ui):
-            yield
-    else:
+    with profile(ui, enabled=ui.configbool('profiling', 'enabled')):
         yield