Restoring pylint speedup for Travis PRs.

Was removed in d922f1d since
the pylint CLI doesn't handle missing files well.

Author: dhermes

run_pylint.py

@@ -125,8 +125,12 @@ def get_files_for_linting():
     a remote branch to diff against.
     """
     diff_base = None
-    # Temporarily turning off origin/master check since file removed.
-    if os.getenv('TRAVIS') is None:
+    if (os.getenv('TRAVIS_BRANCH') == 'master' and
+            os.getenv('TRAVIS_PULL_REQUEST') != 'false'):
+        # In the case of a pull request into master, we want to
+        # diff against HEAD in master.
+        diff_base = 'origin/master'
+    elif os.getenv('TRAVIS') is None:
         # Only allow specified remote and branch in local dev.
         remote = os.getenv('GCLOUD_REMOTE_FOR_LINT')
         branch = os.getenv('GCLOUD_BRANCH_FOR_LINT')