Rebase button should be always enabled.

Bug: Issue 3973
Change-Id: Ie70744d262925f2cc2face9d866b230a56a98537

polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface.js

@@ -301,7 +301,14 @@
 
     getChangeRevisionActions: function(changeNum, patchNum) {
       return this.fetchJSON(
-          this.getChangeActionURL(changeNum, patchNum, '/actions'));
+          this.getChangeActionURL(changeNum, patchNum, '/actions')).then(
+              function(revisionActions) {
+                // The rebase button on change screen is always enabled.
+                if (revisionActions.rebase) {
+                  revisionActions.rebase.enabled = true;
+                }
+                return revisionActions;
+              });
     },
 
     getReviewedFiles: function(changeNum, patchNum) {

polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface_test.html

@@ -222,5 +222,20 @@ limitations under the License.
               element._specialFilePathCompare),
           ['foo/bar.h', 'foo/bar.hpp', 'foo/bar.hxx']);
     });
+
+    test('rebase always enabled', function(done) {
+      var resolveFetchJSON;
+      var fetchJSONStub = sinon.stub(element, 'fetchJSON').returns(
+          new Promise(function(resolve) {
+            resolveFetchJSON = resolve;
+          }));
+      element.getChangeRevisionActions('42', '1337').then(
+          function(response) {
+            assert.isTrue(response.rebase.enabled);
+            fetchJSONStub.restore();
+            done();
+          });
+      resolveFetchJSON({rebase:{}});
+    });
   });
 </script>