Fix encoding of branch names on compare and new merge request page
What does this MR do?
Remove unnecessary encoding from dropdowns on compare and new merge request pages.
Why was this MR needed?
Branches with special characters couldn't be found.
Does this MR meet the acceptance criteria?
- Changelog entry added, if necessary
- Tests added for this feature/bug
- Conform by the code review guidelines
- Has been reviewed by a Frontend maintainer
- Has been reviewed by a Backend maintainer