Fix handling of Gitlab payloads with multiple commits

Gitlab sends multiple commits in the order reversed from Github. As this only broke recently, I suspect that they may have changed the ordering. This change makes the code order-agnostic to hopefully remove the problem going forward.

Fixes #1900

test/test_prepare_trigger.py

@@ -364,6 +364,34 @@ class TestPrepareTrigger(unittest.TestCase):
   def test_gitlab_webhook_nocommit_payload(self):
     self.assertSkipped('gitlab_webhook_nocommit', gl_webhook)
 
+
+  def test_gitlab_webhook_multiple_commits(self):
+    expected = {
+      'commit': u'9a052a0b2fbe01d4a1a88638dd9fe31c1c56ef53',
+      'ref': u'refs/heads/master',
+      'git_url': u'git@gitlab.com:joseph.schorr/some-test-project.git',
+      'commit_info': {
+        'url': u'https://gitlab.com/joseph.schorr/some-test-project/commit/9a052a0b2fbe01d4a1a88638dd9fe31c1c56ef53',
+        'date': u'2016-09-29T15:02:41+00:00',
+        'message': u"Merge branch 'foobar' into 'master'\r\n\r\nAdd changelog\r\n\r\nSome merge thing\r\n\r\nSee merge request !1",
+        'author': {
+          'username': 'josephschorr',
+          'url': 'http://gitlab.com/josephschorr',
+          'avatar_url': 'http://some/avatar/url'
+        },
+      },
+    }
+
+    def lookup_user(_):
+      return {
+        'username': 'josephschorr',
+        'html_url': 'http://gitlab.com/josephschorr',
+        'avatar_url': 'http://some/avatar/url',
+      }
+
+    self.assertSchema('gitlab_webhook_multicommit', expected, gl_webhook, lookup_user=lookup_user)
+
+
   def test_gitlab_webhook_payload_with_lookup(self):
     expected = {
       'commit': u'fb88379ee45de28a0a4590fddcbd8eff8b36026e',