Presubmit: add PRESUBMIT=false to non-master branch changes

PRESUBMIT.py

Index: PRESUBMIT.py
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 6d429dfa1c1ac4116763e36b75641bede6189b45..5599c316c36f2ce1ef973a1b387c34ebfc9ffb6f 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -343,6 +343,8 @@ def PostUploadHook(cl, change, output_api):
     need to be gated on the master branch's tree.
   * Adds 'NOTRY=true' for non master branch changes since trybots do not yet
     work on them.
+  * Adds 'NOPRESUBMIT=true' for non master branch changes since those don't
+    run the presubmit checks.
   """
 
   results = []
@@ -405,6 +407,12 @@ def PostUploadHook(cl, change, output_api):
             output_api.PresubmitNotifyResult(
                 'Trybots do not yet work for non-master branches. '
                 'Automatically added \'NOTRY=true\' to the CL\'s description'))
+      if not re.search(
+          r'^NOPRESUBMIT=true$', new_description, re.M | re.I):
+        new_description += "\nNOPRESUBMIT=true"
+        results.append(
+            output_api.PresubmitNotifyResult(
+                'Branch changes do not run the presubmit checks.'))
 
     # Read and process the HASHTAGS file.
     hashtags_fullpath = os.path.join(change._local_root, 'HASHTAGS')