Added vertical separator to diffs tree list

What does this MR do and why?

Adds vertical separator to the merge request diff files tree list

Screenshots or screen recordings

Before	After

How to set up and validate locally

• View a merge request diff
• Open tree list sidebar

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Closes #335161 (closed)