Clicking label in merge request history goes to wrong page

Steps to reproduce:

Click on a label in the history for a merge request. For example, click the following in gitlab-ce!5190:

Expected behavior:

I expected this to open this page (merge requests with the indicated label), which is the same behavior you get when you click on a label in the fly-out pane of a merge request.

Actual behavior:

The link opened this page (issues with the indicated label).

Impact:

1. In repositories with the issue tracker disabled, the link target gives a 404 error.
2. For labels which only apply to merge requests (e.g. a code review label), the link target does not make sense (wrong context).