Fix "protected branches only" checkbox not set properly at init

The `only_protected_branches` value was being enabled by default
even though the checkbox was default off. In EE, this wasn't a
problem because there was EE-specific code to update the
entire form.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53515

app/assets/javascripts/mirrors/mirror_repos.js

@@ -30,6 +30,7 @@ export default class MirrorRepos {
     this.$password.on('input.updateUrl', () => this.debouncedUpdateUrl());
 
     this.initMirrorSSH();
+    this.updateProtectedBranches();
   }
 
   initMirrorSSH() {

changelogs/unreleased/sh-fix-mirrors-protected-branches.yml

+---
+title: Fix "protected branches only" checkbox not set properly at init
+merge_request: 23409
+author:
+type: fixed