Add debug output to presubmit script for investigating tree check behavior.

PRESUBMIT.py

Index: PRESUBMIT.py
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index fe15157dde7f7e237bb85a77725f6e08b89c3714..5eee2ba31ceb17b5c0576109110901f98274785b 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -66,6 +66,8 @@ def _SkipTreeCheck(input_api, output_api):
   if not input_api.AffectedSourceFiles(
       lambda file: file.LocalPath() == src_version):
     return False
+  if input_api.environ.get('PRESUBMIT_TREE_CHECK') == 'skip':
+    print "Skip tree check requested via environment variable."
   return input_api.environ.get('PRESUBMIT_TREE_CHECK') == 'skip'
 
 
@@ -94,6 +96,7 @@ def CheckChangeOnCommit(input_api, output_api):
   results.extend(input_api.canned_checks.CheckChangeHasDescription(
       input_api, output_api))
   if not _SkipTreeCheck(input_api, output_api):
+    print "Checking if tree is open."
     results.extend(input_api.canned_checks.CheckTreeIsOpen(
         input_api, output_api,
         json_url='http://v8-status.appspot.com/current?format=json'))