Show code coverage in diffs with colored horizontal bar

This would be really nice but hard to do cross-programming languages.

• Phabricator does it for PHP https://secure.phabricator.com/book/phabricator/article/arcanist_coverage/
• Atlassian does it for Java https://www.atlassian.com/software/clover/overview
• Code Climate does it in GitHub https://github.com/marketplace/code-climate