Fixes approval rules not loading correctly for forks

This fixes a bug where approval rules for merge requests would
load the wrong set of rules for forks would send the incorrect
branch name and would load the wrong approval rules for the
selected target branch.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/339918

app/views/shared/issuable/form/_branch_chooser.html.haml

@@ -23,7 +23,7 @@
       = html_escape(_('From %{code_open}%{source_title}%{code_close} into')) % { source_title: source_title, code_open: '<code>'.html_safe, code_close: '</code>'.html_safe }
 
     - if issuable.new_record?
-      %code#js-target-branch-title= target_title
+      %code#js-target-branch-title{ data: { branch_name: @merge_request.target_branch } }= target_title
       &nbsp;
       = link_to _('Change branches'), mr_change_branches_path(issuable)
     - elsif issuable.for_fork?

ee/app/assets/javascripts/approvals/mount_mr_edit.js

@@ -9,8 +9,10 @@ export default function mountApprovalInput(el) {
     return null;
   }
 
+  const targetBranchTitle = document.querySelector('#js-target-branch-title');
   const targetBranch =
-    document.querySelector('#js-target-branch-title')?.textContent ||
+    targetBranchTitle?.dataset?.branchName ||
+    targetBranchTitle?.textContent ||
     document.querySelector('#merge_request_target_branch')?.value;
 
   const store = createStore(