Merge Request versions dropdown cannot navigate to changes against base version

Summary

On a Merge Request with at multiple versions, the second dropdown (mr-version-compare-dropdown) is missing href to the base version.

Steps to reproduce

1. Create a MR with multiple versions
2. Navigate to the "Changes" tab of the MR
3. In the second dropdown of the "Changes between [First Dropdown] and [Second Dropdown]", choose "version 1"
4. Attempt to change the second dropdown back to "master (base)"

Example Project

ChrisRallis/gitlab-11.6-mr-version-example!1

Behavior can be seen at diffs?diff_id=30550374&start_sha=2cf56d93641da86f52011829bcdf2b0c6f797595

What is the current bug behavior?

The second dropdown doesn't allow navigation back to the base revision

What is the expected correct behavior?

The second dropdown allows navigation between the versions it shows

Output of checks

This bug happens on GitLab.com