Much improved file completion.

More efficient, avoids turning paths into stores (and hence real-pathing them)
until one is chosen, prevents selecting directories.
Truncate completions beyond 100.

Actually we show the first 95 and the last 5 to give you an idea of how deep
the rabbit hole goes. But in any case, showing more than that is unperformant
and not super useful. Type more characters to get a more specific completion.