Preserve the base patch when selecting a new patch in the change view

The GR-CHANGE-VIEW element responds to changes in "Patch Set" select
input by constructing and loading a new router URL. However, if the base
patch did not have PARENT selected, it would not be included in the new
route, effectively resetting the base patch to PARENT whenever the patch
is changed.

With this change, the base patch selection is included in such router
URLs.

Bug: Issue 5254
Change-Id: I757bba23f6e23d4b63bc57183c65d2365cb88f5f

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

diff --git a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
index d7c0407..53119c3 100644
--- a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
+++ b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
@@ -511,7 +511,7 @@
 
     /**
      * Change active patch to the provided patch num.
-     * @param {int} patchNum the patchn number to be viewed.
+     * @param {number} patchNum the patchn number to be viewed.
      */
     _changePatchNum: function(patchNum) {
       var currentPatchNum;
@@ -521,11 +521,14 @@
       } else {
         currentPatchNum = this._computeLatestPatchNum(this._allPatchSets);
       }
-      if (patchNum === currentPatchNum) {
+      if (patchNum === currentPatchNum &&
+          this._patchRange.basePatchNum === 'PARENT') {
         page.show(this.changePath(this._changeNum));
         return;
       }
-      page.show(this.changePath(this._changeNum) + '/' + patchNum);
+      var patchExpr = this._patchRange.basePatchNum === 'PARENT' ? patchNum :
+          this._patchRange.basePatchNum + '..' + patchNum;
+      page.show(this.changePath(this._changeNum) + '/' + patchExpr);
     },
 
     _computeChangePermalink: function(changeNum) {

polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_test.html

diff --git a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_test.html b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_test.html
index d20c194..ac07952 100644
--- a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_test.html
+++ b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_test.html
@@ -499,6 +499,35 @@
       assert.isTrue(reloadStub.calledTwice);
     });
 
+    test('include base patch when not parent', function() {
+      element._changeNum = '42';
+      element._patchRange = {
+        basePatchNum: '2',
+        patchNum: '3',
+      };
+      element._change = {
+        change_id: 'Iad9dc96274af6946f3632be53b106ef80f7ba6ca',
+        revisions: {
+          rev2: {_number: 2},
+          rev1: {_number: 1},
+          rev13: {_number: 13},
+          rev3: {_number: 3},
+        },
+        status: 'NEW',
+        labels: {},
+      };
+
+      var showStub = sandbox.stub(page, 'show');
+
+      element._changePatchNum(13);
+      assert(showStub.lastCall.calledWithExactly('/c/42/2..13'));
+
+      element._patchRange.basePatchNum = 'PARENT';
+
+      element._changePatchNum(3);
+      assert(showStub.lastCall.calledWithExactly('/c/42/3'));
+    });
+
     test('related changes are updated after rebase', function(done) {
       sandbox.stub(element, '_reload',
           function() { return Promise.resolve(); });