Add placeholder for the "Default Branch" field in the create repo wizard

To make it clear that setting a default branch is optional and if you
omit to set it the default branch will be master.

Release-Notes: Added placeholder text for "Default Branch" field in the
create repo wizard
Feature: Issue 296889346
Change-Id: Ie61ed9e5fda9f06f7fc8f1fb7ef92ea9aa5be865

polygerrit-ui/app/elements/admin/gr-create-repo-dialog/gr-create-repo-dialog.ts

diff --git a/polygerrit-ui/app/elements/admin/gr-create-repo-dialog/gr-create-repo-dialog.ts b/polygerrit-ui/app/elements/admin/gr-create-repo-dialog/gr-create-repo-dialog.ts
index 1a70f2b..ca7a78f 100644
--- a/polygerrit-ui/app/elements/admin/gr-create-repo-dialog/gr-create-repo-dialog.ts
+++ b/polygerrit-ui/app/elements/admin/gr-create-repo-dialog/gr-create-repo-dialog.ts
@@ -112,12 +112,15 @@
           </section>
           <section>
             <span class="title">Default Branch</span>
-            <iron-input
-              .bindValue=${convertToString(this.defaultBranch)}
-              @bind-value-changed=${this.handleBranchNameBindValueChanged}
-            >
-              <input id="defaultBranchNameInput" autocomplete="off" />
-            </iron-input>
+            <span class="value">
+              <gr-autocomplete
+                id="defaultBranchNameInput"
+                .text=${convertToString(this.defaultBranch)}
+                .placeholder=${"Optional, defaults to 'master'"}
+                @text-changed=${this.handleBranchNameBindValueChanged}
+              >
+              </gr-autocomplete>
+            </span>
           </section>
           <section>
             <span class="title">Rights inherit from</span>

polygerrit-ui/app/elements/admin/gr-create-repo-dialog/gr-create-repo-dialog_test.ts

diff --git a/polygerrit-ui/app/elements/admin/gr-create-repo-dialog/gr-create-repo-dialog_test.ts b/polygerrit-ui/app/elements/admin/gr-create-repo-dialog/gr-create-repo-dialog_test.ts
index 63b8c06..f8476e6 100644
--- a/polygerrit-ui/app/elements/admin/gr-create-repo-dialog/gr-create-repo-dialog_test.ts
+++ b/polygerrit-ui/app/elements/admin/gr-create-repo-dialog/gr-create-repo-dialog_test.ts
@@ -39,9 +39,9 @@
             </section>
             <section>
               <span class="title"> Default Branch </span>
-              <iron-input>
-                <input autocomplete="off" id="defaultBranchNameInput" />
-              </iron-input>
+              <span class="value">
+                <gr-autocomplete id="defaultBranchNameInput"> </gr-autocomplete>
+              </span>
             </section>
             <section>
               <span class="title"> Rights inherit from </span>