Notice when buffet source has been modified

[abraham]

Some tools automatically modify dotfiles to get included in your path. These modifications should either be made to *.local files or upstreamed to buffet if they are common enough. It would be good if when opening a new tab/window there was a warning if buffet source is dirty.

[wstrinz]

This would definitely be a nice feature. For starters we could just trigger a warning when git diff ~/.bashrc returned a nonzero exit code or something like that.