Disabling auto-fill-mode for git-commit-mode

Because of GitHub's stupid decision in their flavor of Markdown, newlines in commit messages are actually interpreted as line breaks. So every programmer that uses a proper editor which automatically breaks their text 80 characters has weird looking commits. This disables Magit's auto-fill hook, and adds visual-line-mode so that visually, the commit message buffer looks the same. GFM needs to GTFO. Even Ghost blog is doing it too.