WIP: Display both HEAD and base options in the merge request comparisons dropdown

What does this MR do?

Fix bug where HEAD overrides base such that only HEAD is shown.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Related to #209071 (closed)