Add PG UI support for new changes with base commit

This allows users to specify a non-HEAD base commit
when creating a new change from the web UI.

Bug: Issue 8857
Change-Id: Ic585fafacc4bf21cd1d8bd129e889b42180b80a7

polygerrit-ui/app/elements/admin/gr-create-change-dialog/gr-create-change-dialog.html

@@ -72,6 +72,17 @@ limitations under the License.
             </gr-autocomplete>
           </span>
         </section>
+        <section class$="[[_computeBranchClass(baseChange)]]">
+          <span class="title">Provide base commit sha1 for change</span>
+          <span class="value">
+            <input
+                is="iron-input"
+                id="baseCommitInput"
+                maxlength="40"
+                placeholder="(optional)"
+                bind-value="{{baseCommit}}">
+          </span>
+        </section>
         <section>
           <span class="title">Enter topic for new change</span>
           <span class="value">

polygerrit-ui/app/elements/admin/gr-create-change-dialog/gr-create-change-dialog.js

@@ -37,6 +37,7 @@
         },
       },
       baseChange: String,
+      baseCommit: String,
       privateByDefault: String,
       canCreate: {
         type: Boolean,
@@ -73,7 +74,8 @@
       const isPrivate = this.$.privateChangeCheckBox.checked;
       const isWip = true;
       return this.$.restAPI.createChange(this.repoName, this.branch,
-          this.subject, this.topic, isPrivate, isWip, this.baseChange)
+          this.subject, this.topic, isPrivate, isWip, this.baseChange,
+          this.baseCommit)
           .then(changeCreated => {
             if (!changeCreated) { return; }
             Gerrit.Nav.navigateToChange(changeCreated);

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

@@ -1410,9 +1410,10 @@
      * @param {boolean=} opt_isPrivate
      * @param {boolean=} opt_workInProgress
      * @param {string=} opt_baseChange
+     * @param {string=} opt_baseCommit
      */
     createChange(project, branch, subject, opt_topic, opt_isPrivate,
-        opt_workInProgress, opt_baseChange) {
+        opt_workInProgress, opt_baseChange, opt_baseCommit) {
       return this.send('POST', '/changes/', {
         project,
         branch,
@@ -1421,6 +1422,7 @@
         is_private: opt_isPrivate,
         work_in_progress: opt_workInProgress,
         base_change: opt_baseChange,
+        base_commit: opt_baseCommit,
       }).then(response => this.getResponseObject(response));
     },