Add a lint to check that we're not committing any conflicts

The following discussion from gitlab-ee!2538 should be addressed:

• @godfat started a discussion: (+1 comment)

  Whoops! We should be more careful merging from CE to EE

  @rymai What do you think if we have a lint to check if there's no <<<<<<< HEAD across the codebase? I usually search against that to find unresolved conflicts, and so far it's 100% accurate.