Allow users to compare branches on a read-only instance

What does this MR do?

Change the link to compare branches on the repository branches page to perform a GET request allowing users to compare branches on a read-only instance.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/9841

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [-] Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer