Re-enable eslint in `commits.js` file

app/assets/javascripts/commits.js has eslint disabled as a consequence of the conversion from Coffeescript into plain JS

Task:

1. Re-enable eslint and fix the broken rules

Note: In similar refactors we've transformed it into an ES6 class: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15503/diffs