"function --argument" is not a thing, it's "--argument-names". This only
accidentally works because our getopt is awful and allows abbreviated
long options.
Similarly, one argparse test used "--d" instead of "-d" or "--def".
The default completions that autojump ships with for fish are broken
(emitting output like "1\___\#...") as they use hackes to work around
the previous lack of `complete -k`. The history-based autojump
completions fully replace it.
j does not have any "logical" source of completions, but it almost often
called with arguments that have been seen before (since it is used to
jump to favorite/recent directories). We can search the history for
possible completions and use those.
This is an example of the behavior mentioned in #4344 as a possible
enhancement for fish 3.0, where completions can be provided from history
if none are otherwise found.
