Bug 1182677 - Support calling a function during mach command dispatch; r=smacleod

This will give us the ability to execute custom code at command dispatch
time. We can use this for global tests before dispatch.

--HG--
extra : commitid : 5vfmSqOis4W
extra : rebase_source : c285b16007c4b604b164d079a275198c8760e480

python/mach/mach/registrar.py

@@ -61,6 +61,11 @@ class MachRegistrar(object):
         if handler.pass_context and not context:
             raise Exception('mach command class requires context.')
 
+        if context:
+            prerun = getattr(context, 'pre_dispatch_handler', None)
+            if prerun:
+                prerun(context, handler, args=kwargs)
+
         if handler.pass_context:
             instance = cls(context)
         else: