Disable create branch / merge request button until validation completed

app/assets/javascripts/create_merge_request_dropdown.js

@@ -311,6 +311,7 @@ export default class CreateMergeRequestDropdown {
   }
 
   onChangeInput(event) {
+    this.disable();
     let target;
     let value;
 

changelogs/unreleased/fix-incorrect-new-branch-name-from-issue.yml

+---
+title: Fix incorrect new branch name from issue
+merge_request: 20677
+author: Lee Tickett
+type: fixed