React to changes in .gitignore without restarting

[naholyr]

If you add/remove/modify .gitignore it won't have any impact on the file list until you restart. We should watch for this, and when such change is detected:

• We could simply automatically restart the server (socket.io's auto-reconnect will make it transparent, but we have to ensure tunnels are as tolerant), that's a good first step
• We could then be smarter, update tree accordingly, and push it to sockets