wrap lines at 79 characters

[Trevor Keller]

The GitHub diff interface wraps lines at 80 characters. Since we use 100 characters as the max width, this makes for ugly diffs with lots of continued lines. To improve the clarity of these diffs, as well as readability in terminal text editors, we should back away from this edge, and wrap our content at 79 characters.

This is easily done with emacs, but the change would touch almost every file in the repository, so this should be coordinated with other efforts to minimize merge conflicts and the number of branch rebases to be done.