Do not show "Patch file" if change does not have a parent

Bug: Issue 8495
Change-Id: I4047e600994beedf35562ff4c4f9ea9632808920

polygerrit-ui/app/elements/change/gr-download-dialog/gr-download-dialog.html

@@ -19,8 +19,8 @@ limitations under the License.
 
 <link rel="import" href="../../../behaviors/gr-patch-set-behavior/gr-patch-set-behavior.html">
 <link rel="import" href="../../../behaviors/rest-client-behavior/rest-client-behavior.html">
-<link rel="import" href="../../shared/gr-download-commands/gr-download-commands.html">
 <link rel="import" href="../../../styles/shared-styles.html">
+<link rel="import" href="../../shared/gr-download-commands/gr-download-commands.html">
 
 <dom-module id="gr-download-dialog">
   <template>
@@ -87,7 +87,7 @@ limitations under the License.
           selected-scheme="{{_selectedScheme}}"></gr-download-commands>
     </section>
     <section class="flexContainer">
-      <div class="patchFiles">
+      <div class="patchFiles" hidden="[[_computeHidePatchFile(change, patchNum)]]" hidden>
         <label>Patch file</label>
         <div>
           <a

polygerrit-ui/app/elements/change/gr-download-dialog/gr-download-dialog.js

@@ -138,6 +138,17 @@
       return shortRev + '.diff.' + (opt_zip ? 'zip' : 'base64');
     },
 
+    _computeHidePatchFile(change, patchNum) {
+      for (const rev of Object.values(change.revisions || {})) {
+        if (this.patchNumEquals(rev._number, patchNum)) {
+          const parentLength = rev.commit && rev.commit.parents ?
+                rev.commit.parents.length : 0;
+          return parentLength == 0;
+        }
+      }
+      return false;
+    },
+
     _computeArchiveDownloadLink(change, patchNum, format) {
       return this.changeBaseURL(change.project, change._number, patchNum) +
           '/archive?format=' + format;

polygerrit-ui/app/elements/change/gr-download-dialog/gr-download-dialog_test.html

@@ -45,6 +45,9 @@ limitations under the License.
       revisions: {
         '34685798fe548b6d17d1e8e5edc43a26d055cc72': {
           _number: 1,
+          commit: {
+            parents: [],
+          },
           fetch: {
             repo: {
               commands: {
@@ -105,6 +108,9 @@ limitations under the License.
       revisions: {
         '34685798fe548b6d17d1e8e5edc43a26d055cc72': {
           _number: 1,
+          commit: {
+            parents: [],
+          },
           fetch: {},
         },
       },
@@ -188,5 +194,25 @@ limitations under the License.
       assert.equal(element._computeShowDownloadCommands([]), 'hidden');
       assert.equal(element._computeShowDownloadCommands(['test']), '');
     });
+
+    test('_computeHidePatchFile', () => {
+      const patchNum = '1';
+
+      const change1 = {
+        revisions: {
+          r1: {_number: 1, commit: {parents: []}},
+        },
+      };
+      assert.isTrue(element._computeHidePatchFile(change1, patchNum));
+
+      const change2 = {
+        revisions: {
+          r1: {_number: 1, commit: {parents: [
+            {commit: 'p1'},
+          ]}},
+        },
+      };
+      assert.isFalse(element._computeHidePatchFile(change2, patchNum));
+    });
   });
 </script>