Don't add PR title in commit message on squash

Github will use the PR title as the commit subject for squash merges, so
we don't need include the title once again in the commit description.

Change-Id: Id5a00701c236235f5a49abd025bcfad1b2da916c

tests/fakegithub.py

@@ -730,7 +730,7 @@ class FakeGithubSession(object):
                     'message': 'Merge not allowed because of fake reason',
                 }
                 return FakeResponse(data, 405, 'Method not allowed')
-            pr.setMerged(json["commit_message"])
+            pr.setMerged(json.get("commit_message", ""))
             return FakeResponse({"merged": True}, 200)
 
         return FakeResponse(None, 404)

tests/unit/test_github_driver.py

@@ -1430,7 +1430,9 @@ class TestGithubDriver(ZuulTestCase):
         repo._set_branch_protection(
             'master', contexts=['tenant-one/check', 'tenant-one/gate'])
 
-        A = self.fake_github.openFakePullRequest('org/project', 'master', 'A')
+        pr_description = "PR description"
+        A = self.fake_github.openFakePullRequest('org/project', 'master', 'A',
+                                                 body_text=pr_description)
         self.fake_github.emitEvent(A.getPullRequestOpenedEvent())
         self.waitUntilSettled()
 
@@ -1448,6 +1450,9 @@ class TestGithubDriver(ZuulTestCase):
         merges = [report for report in self.fake_github.github_data.reports
                   if report[2] == 'merge']
         assert (len(merges) == 1 and merges[0][3] == 'squash')
+        # Assert that we won't duplicate the PR title in the merge
+        # message description.
+        self.assertEqual(A.merge_message, pr_description)
 
     @simple_layout('layouts/basic-github.yaml', driver='github')
     def test_invalid_event(self):

zuul/driver/github/githubreporter.py

@@ -193,13 +193,13 @@ class GithubReporter(BaseReporter):
             self.log.warning('Merge mode %s not supported by Github', mode)
             raise MergeFailure('Merge mode %s not supported by Github' % mode)
 
-        merge_mode = self.merge_modes[merge_mode]
         project = item.change.project.name
         pr_number = item.change.number
         sha = item.change.patchset
         log.debug('Reporting change %s, params %s, merging via API',
                   item.change, self.config)
-        message = self._formatMergeMessage(item.change)
+        message = self._formatMergeMessage(item.change, merge_mode)
+        merge_mode = self.merge_modes[merge_mode]
 
         for i in [1, 2]:
             try:
@@ -319,10 +319,13 @@ class GithubReporter(BaseReporter):
             self.connection.unlabelPull(project, pr_number, label,
                                         zuul_event_id=item.event)
 
-    def _formatMergeMessage(self, change):
+    def _formatMergeMessage(self, change, merge_mode):
         message = []
-        if change.title:
-            message.append(change.title)
+        # For squash merges we don't need to add the title to the body
+        # as it will already be set as the commit subject.
+        if merge_mode != model.MERGER_SQUASH_MERGE:
+            if change.title:
+                message.append(change.title)
         if change.body_text:
             message.append(change.body_text)
         merge_message = "\n\n".join(message)