Report dequeued changes via Github checks API

This patch provides the general functionality to allow reporting of
dequeued items and makes use of that in the Github checks API.
This reporting will only apply if the item wasn't a success or failure.

Change-Id: I1297da4d1708908c6b179110479fe0450e5550fe

zuul/reporter/__init__.py

@@ -123,7 +123,8 @@ class BaseReporter(object, metaclass=abc.ABCMeta):
             'failure': self._formatItemReportFailure,
             'merge-failure': self._formatItemReportMergeFailure,
             'no-jobs': self._formatItemReportNoJobs,
-            'disabled': self._formatItemReportDisabled
+            'disabled': self._formatItemReportDisabled,
+            'dequeue': self._formatItemReportDequeue,
         }
         return format_methods[self._action]
 
@@ -208,6 +209,12 @@ class BaseReporter(object, metaclass=abc.ABCMeta):
         else:
             return self._formatItemReport(item)
 
+    def _formatItemReportDequeue(self, item, with_jobs=True):
+        msg = item.pipeline.dequeue_message
+        if with_jobs:
+            msg += '\n\n' + self._formatItemReportJobs(item)
+        return msg
+
     def _getItemReportJobsFields(self, item):
         # Extract the report elements from an item
         config = self.connection.sched.config