gerrit: trim messages to "human length"

The "message" field of a gerrit response (i.e. the bit that has "Zuul
encountered a syntax error while parsing its configuration ...") has a
limit in Gerrit of "change.commentSizeLimit" which is by default 16KiB
[1].

If you have a syntax error in a very long section, say the middle of
the project: section of OpenDev system-config, you can actually
overflow this length.  Gerrit rejects the update and as an end-user
you don't get any clue why.

This truncates the message in the gerrit driver if it's going to
overflow.

[1] Note change.robotCommentSizeLimit is larger, but that's not how
syntax messages are reported.

Change-Id: I2f14e734ef5f9f203b14556c7d2c8ed1ad052319

tests/unit/test_gerrit.py

@@ -184,6 +184,28 @@ class TestGerritWeb(ZuulTestCase):
                                        'username': 'jenkins'}}
         )
 
+    def test_message_too_long(self):
+        in_repo_conf = textwrap.dedent(
+            """
+            - job:
+                name: garbage-job
+                %s
+                garbage: True
+            """
+        ) % ('\n' * 16384)
+
+        file_dict = {'.zuul.yaml': in_repo_conf}
+        A = self.fake_gerrit.addFakeChange('org/project', 'master', 'A',
+                                           files=file_dict)
+        self.fake_gerrit.addEvent(A.getPatchsetCreatedEvent(1))
+        self.waitUntilSettled()
+
+        self.assertEqual(A.patchsets[0]['approvals'][0]['value'], "-1")
+        self.assertEqual(A.patchsets[0]['approvals'][0]['__tag'],
+                         "autogenerated:zuul:check")
+        self.assertIn('... (truncated)',
+                      A.messages[0])
+
     def test_dependent_dynamic_line_comment(self):
         in_repo_conf = textwrap.dedent(
             """