Graph of the code coverage percentage for a specific branch

Reporting feedback from this thread: https://about.gitlab.com/2015/11/22/gitlab-8-2-released/#comment-2376412829

Before the 8.2 release, it was possible to have a tab for a specific branch in the CI page. As the percentage of code coverage was displayed in this page, it was easy to monitor the evolution of the coverage.

There is no longer a tab for a specific branch. So I'd like to be able to select a few branches and to have graphs that show their percentage of code coverage.

cc @ayufan