Set topic by Change-Id

When setting a topic, GR-CHANGE-METADATA would call the set-topic API
endpoint identifying the change using the following format:

    <project>~<branch>~<Change-Id>

When the project or branch contained characters that needed to be URI-
encoded, the endpoint would fail to identify the change, resulting in a
404. This API behavior is encoded in Issue 4746.

The workaround in this change is to instead identify the change by the
shorter Change-Id format (`Ie<hex ...>`), which needs no additional
encoding.

Bug: Issue 4555
Change-Id: I00181387bc2a85dfea90b9db46fb017749963579

polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata.js

@@ -88,7 +88,7 @@
 
     _handleTopicChanged: function(e, topic) {
       if (!topic.length) { topic = null; }
-      this.$.restAPI.setChangeTopic(this.change.id, topic);
+      this.$.restAPI.setChangeTopic(this.change.change_id, topic);
     },
 
     _computeTopicReadOnly: function(mutable, change) {

polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata_test.html

@@ -85,6 +85,8 @@ limitations under the License.
         sandbox.stub(element, '_computeValueTooltip').returns('');
         sandbox.stub(element, '_computeTopicReadOnly').returns(true);
         element.change = {
+          change_id: 'the id',
+          topic: 'the topic',
           status: 'NEW',
           submit_type: 'CHERRY_PICK',
           labels: {
@@ -145,6 +147,13 @@ limitations under the License.
           done();
         });
       });
+
+      test('changing topic calls setChangeTopic', function() {
+        var topicStub = sandbox.stub(element.$.restAPI, 'setChangeTopic',
+            function() {});
+        element._handleTopicChanged({}, 'the new topic');
+        assert.isTrue(topicStub.calledWith('the id', 'the new topic'));
+      });
     });
   });
 </script>