Magit support

Implementation credit goes to @binarin, but I'm requesting a merge because I find it useful. Admittedly, special-casing magit-status-mode buffers smells a little wrong, but the same thing is done with dired buffers. Ideally, there'd be some macro that provides a generic way to add special case buffers to the tests in  `eproject--buffer-file-name`, but I haven't spent any time on it because this works for me.