Bug 1420449: Add mach taskgraph -F/--fast r=ahal

Initially this will skip toolchain task optimizations, which avoids hashing
local directory contents and speeds up taskgraph generation by about 25%.

MozReview-Commit-ID: B4LB5BV86nw

taskcluster/mach_commands.py

@@ -51,7 +51,9 @@ class ShowTaskGraphSubCommand(SubCommand):
                             "index (a.k.a. optimize the graph)"),
             CommandArgument('--tasks-regex', '--tasks', default=None,
                             help="only return tasks with labels matching this regular "
-                            "expression.")
+                            "expression."),
+            CommandArgument('-F', '--fast', dest='fast', default=False, action='store_true',
+                            help="enable fast task generation for local debugging.")
 
         ]
         for arg in args:
@@ -316,6 +318,9 @@ class MachCommands(MachCommandBase):
         import taskgraph.parameters
         import taskgraph.target_tasks
         import taskgraph.generator
+        import taskgraph
+        if options['fast']:
+            taskgraph.fast = True
 
         try:
             self.setup_logging(quiet=options['quiet'], verbose=options['verbose'])