ui: add a variable to control whether hooks should be called

So hooks can be temporarily disabled.

mercurial/hook.py

@@ -138,6 +138,9 b' def redirect(state):'
     _redirect = state
 
 def hook(ui, repo, name, throw=False, **args):
+    if not ui.callhooks:
+        return False
+
     r = False
 
     oldstdout = -1

mercurial/ui.py

@@ -19,6 +19,7 b' class ui(object):'
         self._ucfg = config.config() # untrusted
         self._trustusers = set()
         self._trustgroups = set()
+        self.callhooks = True
 
         if src:
             self.fout = src.fout
@@ -31,6 +32,7 b' class ui(object):'
             self._trustusers = src._trustusers.copy()
             self._trustgroups = src._trustgroups.copy()
             self.environ = src.environ
+            self.callhooks = src.callhooks
             self.fixconfig()
         else:
             self.fout = sys.stdout