Adds saner behavior for git try -t foo to not fail in a inexplicable way. Enforces that each builde…

presubmit_support.py

Index: presubmit_support.py
diff --git a/presubmit_support.py b/presubmit_support.py
index 5bd049bcda67ece3b827d202a05cc1db379b1171..0b72f4656b72480d28e72283faa7997749a090c5 100755
--- a/presubmit_support.py
+++ b/presubmit_support.py
@@ -926,6 +926,9 @@ class GetTrySlavesExecuter(object):
         if item != item.strip():
           raise PresubmitFailure(
               'Try slave names cannot start/end with whitespace')
+        if ',' in item:
+          raise PresubmitFailure(
+              'Do not use \',\' separated builder or test names: %s' % item)
     else:
       result = []
     return result