gr-change-view: Skip loading mergeable if already known

The asynchronous mergeable call was added in: I01d03de9f2, and was
partly reverted in major refactoring change: I01c87f6f6d7. The part to
skip retrieval of mergeable attribute if already provided by changes
detail endpoint was missed to be reverted.

Bug: Issue 14577
Change-Id: I55e3f32761f0eb484c3b8c3799fd0a6568976017
(cherry picked from commit acb76bdfa491ba7141019c0b8b292e69638a6ab9)

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

@@ -1871,6 +1871,12 @@ class GrChangeView extends mixinBehaviors( [
       return Promise.resolve();
     }
 
+    // If mergeable bit was already returned in detail REST endpoint, use it.
+    if (this._change.mergeable !== undefined) {
+      this._mergeable = this._change.mergeable;
+      return Promise.resolve();
+    }
+
     this._mergeable = null;
     return this.$.restAPI.getMergeable(this._changeNum).then(m => {
       this._mergeable = m.mergeable;