Fix missing file name in diff view when file has no change but comments

As getChangedFiles API only returns files with changes while
files without changes but comments still accessible from diff view,
should always add the current file to the file list.

Bug: Issue 11725
Change-Id: I0769ce7220b16c21af989fa94eb06775d6a63cdf
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js
index 5087d74..72527a1 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js
@@ -290,6 +290,12 @@
       return this.$.restAPI.getChangeFilePathsAsSpeciallySortedArray(
           changeNum, patchRange).then(files => {
         this._fileList = files;
+
+        // in case current file is not in changed files
+        // (file has no change but has comments)
+        if (this._path && !this._fileList.includes(this._path)) {
+          this._fileList.push(this._path);
+        }
       });
     }
 
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html
index 29cc950..bd172a5 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html
@@ -433,6 +433,23 @@
       });
     });
 
+    suite('unchanged file', () => {
+      test('unchanged file should be included in _fileList', done => {
+        element._path = 'aaa.txt';
+        // trigger reload on files
+        element._changeNum = '123';
+        element._patchRange = {
+          basePatchNum: PARENT,
+          patchNum: '1',
+        };
+        assert.isFalse(element._fileList.includes('aaa.txt'));
+        flush(() => {
+          assert.isTrue(element._fileList.includes('aaa.txt'));
+          done();
+        });
+      });
+    });
+
     suite('url params', () => {
       setup(() => {
         sandbox.stub(