Ask to merge settings when registering repo

The `add' command will ask the user if they would like to merge their
settings with the newly registered repository's if the current input
is a tty and if the user has specified some settings.

The default for this is `#t', so pressing <RET> when presented with
this question will merge the settings, however if the user is never
asked (because the input is not a tty) no merge will happen.