Improve tramp candidates completion

[clemera]

As per comment by @mathrick here:

Things I've observed:

• I can't tell if this is a bug or intended design, but if I connect through /ssh: with an explicit username, (ie. /ssh:[email protected]:), then mathrick@ and example.com will be saved separately into the list of candidates, so the next time I type /ssh:, I will have the following candidates:
• /ssh:mathrick@
• /ssh:example.com
• /ssh:somewhere-else.net

But not /ssh:[email protected]. And if I select the mathrick@, then the list becomes:

• /ssh:[email protected]
• /ssh:[email protected]

Ie. it seems to be a cartesian product of the username with all possible hostnames, even if I've never actually connected with that particular combination.

• If I type /ssh:example.com:<TAB> (that is, using default username), then go back and edit it to /ssh:[email protected]:<TAB> (with an explicit username), the list of saved candidates will now include:
• /ssh:mexample.com:
• /ssh:maexample.com:
• /ssh:matexample.com:
• /ssh:mathexample.com:
• /ssh:mathrexample.com:
• /ssh:mathriexample.com:
• /ssh:mathricexample.com:
• /ssh:mathrickexample.com:

These issues aren't related to Selectrum but seem to be a general issue with tramp completions as these results are returned by the built-in file table even though those intermediate steps aren't passed to it by Selectrum and even when skipping Selectrums post command hook entirely those completions are still added.

[manuel-uberti]

Would it make sense to report it as bug directly on Emacs or maybe ask some clarification on emacs-devel?

[glallen01]

Removing my addition here as I think it was addressed by #487. Thank you!