roll-dep: Run gclient.py rather than gclient

gclient calls into update_depot_tools, where gclient.py does not.

Change-Id: Idabf0fd613ee241460fbeb6b897a79b084bbc4e2
Reviewed-on: https://chromium-review.googlesource.com/1000786Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Eric Boren <borenet@chromium.org>

roll_dep.py

@@ -71,7 +71,9 @@ def should_show_log(upstream_url):
 
 
 def get_gclient_root():
-  return check_output(['gclient', 'root']).strip()
+  gclient = os.path.join(
+      os.path.dirname(os.path.abspath(__file__)), 'gclient.py')
+  return check_output([sys.executable, gclient, 'root']).strip()
 
 
 def get_deps(root):