Bug 1423887 - [tryselect] Enable taskgraph.fast when generating tasks from |mach try|, r=catlee

This speeds up taskgraph generation by ~6 seconds on my machine. Future
improvements are also planned for 'fast' mode.

MozReview-Commit-ID: CLORvLXuV8y

--HG--
extra : rebase_source : a59dc5f166eff15e5031f5736eadac41dcd46ffe

tools/tryselect/tasks.py

@@ -11,6 +11,7 @@ from mozboot.util import get_state_dir
 from mozbuild.base import MozbuildObject
 from mozpack.files import FileFinder
 
+import taskgraph
 from taskgraph.generator import TaskGraphGenerator
 from taskgraph.parameters import (
     ParameterMismatch,
@@ -68,6 +69,7 @@ def generate_tasks(params, full, root):
         print(PARAMETER_MISMATCH.format(e.args[0]))
         sys.exit(1)
 
+    taskgraph.fast = True
     cwd = os.getcwd()
     os.chdir(build.topsrcdir)