Fix try jobs not within a gclient checkout

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/3331004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@58286 0039d316-1c4b-4281-b951-d872f2087c98

trychange.py

@@ -163,6 +163,8 @@ class SCM(object):
       root = os.path.abspath(self.gclient_root)
     else:
       root = gclient_utils.FindGclientRoot(cur)
+    if not root:
+      root = cur
     assert cur.startswith(root), (root, cur)
     while cur.startswith(root):
       filepath = os.path.join(cur, filename)