Allow PRESUBMIT run pcg on trybuilder.

PRESUBMIT.py

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Top-level presubmit script.
 
 See https://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts for
 details on the presubmit API built into depot_tools.
 """
 
@@ skipping 14 matching lines @@
     pcg = os.path.join(p, exe)
     if os.access(pcg, os.X_OK):
       break
   else:
     return [
       error_type(
         'pre-commit-go executable (pcg) could not be found in PATH. All Go '
         'checks are skipped. See https://github.com/maruel/pre-commit-go.')
     ]
 
-  # pcg can figure out what files to check on its own based on upstream ref.
   cmd = [pcg, 'run', '-m', ','.join(pcg_mode)]
   if input_api.verbose:
     cmd.append('-v')
+  # pcg can figure out what files to check on its own based on upstream ref,
+  # but on PRESUBMIT try builder upsteram isn't set, and it's just 1 commit.
+  if os.getenv('PRESUBMIT_BUILDER', ''):
+    cmd.extend(['-r', 'HEAD~1'])
   return input_api.RunTests([
     input_api.Command(
       name='pre-commit-go: %s' % ', '.join(pcg_mode),
       cmd=cmd,
       kwargs={},
       message=error_type),
   ])
 
 
 def header(input_api):
@@ skipping 48 matching lines @@
   results = CommonChecks(input_api, output_api)
   results.extend(input_api.canned_checks.CheckChangeHasDescription(
       input_api, output_api))
   results.extend(input_api.canned_checks.CheckDoNotSubmitInDescription(
       input_api, output_api))
   results.extend(input_api.canned_checks.CheckDoNotSubmitInFiles(
       input_api, output_api))
   results.extend(PreCommitGo(
       input_api, output_api, ['lint', 'pre-commit', 'pre-push']))
   return results