Don't cycle through full completion list if $fish_complete_list == 0

I have a habit of pressing `tab` more than once when completing files. I know this is irrational, the reasons are probably some combination of idle habit, impatience, slight OCD, and distrust that the first tab was recognised. I doubt I'm alone, but I don't have any data to back that up.

Anyway, I would love to be able to disable the "start completing full ambiguous matches from the completion list" on tab presses past the first. I have never intentionally triggered this behaviour, but because of the above I do it accidentally quite often. When that happens, I have to remember what I had written and manually backspace the additional stuff fish inserted, since there's no way to undo one of these completions cleanly. I thought I'd grow out of the habit with usage, but I've been using fish full-time for months now, and it still gets me regularly.

This patch disables the full (ambiguous) completion behaviour if `$fish_complete_list` is set to `"0"`