Homogenize revert and cherry-pick button styles generated by commits_helper.rb

What does this MR do?

Prevents inconsistent "revert" and "cherry-pick" button styles from being generated based on whether a user has write access to a project.

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

Why was this MR needed?

Screenshots (if relevant)

Before (no access on left, has access on right):

After (no access on left, has access on right):

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 #25225 (closed)