bug#44611: Prefix arg for xref-goto-xref

[Juri Linkov]

> The original justification for this binding (authored by Joao) was that by
> using it we indicate that the Xref buffer is used for "completion" (picking
> one result), rather then iterating over multiple matches.
>
> That's why it's TAB, because "TAB completion", apparently. Overall, it's
> not obvious, but it kinda makes sense.

This interpretation of picking one result doesn't fit into my workflow:
I use RET to iterate over multiple matches, then need to close the
*xref* window after visiting the last match, so TAB makes no sense
in this case.  I'd expect TAB rather to iterate over multiple matches,
i.e. like TAB in browsers go to the next match.  Even in the *Completions*
buffer TAB moves to the next completion.  And in icomplete-mode
the closest analogy to picking one result is 'C-j'
(icomplete-force-complete-and-exit).