Change the approach to check if patches apply cleanly

What does this MR do?

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #23372 (closed)

lib/gitlab/ee_compat_check.rb

@@ -107,7 +107,7 @@ def generate_patch(branch, filepath)
       raise "#{branch} is too far behind master, please rebase it!" if depth > 500
 
       step("Generating the patch against master")
-      output, status = Gitlab::Popen.popen(%w[git format-patch origin/master --stdout])
+      output, status = Gitlab::Popen.popen(%w[git format-patch FETCH_HEAD --stdout])
       throw(:halt_check, :ko) unless status.zero?
 
       File.open(filepath, 'w+') { |f| f.write(output) }