Update rsync completions

[Jakeler]

Description

This adds completions for the newer (and very useful) info+debug flags, parsed from the help output:

Also many other options that were missing. Based on the current rsync 3.2.3 man page.

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

[faho]

A space after the ; would be nice.

(also this does user:group, so maybe (__fish_complete_users):(__fish_complete_groups) might be helpful, or more complicated logic to only complete the group after the user - see __fish_complete_user_at_hosts for something similar)

[Jakeler]

(__fish_complete_users):(__fish_complete_groups) unfortunately does not work in my testing, if I type : and hit TAB it removes the whole option? Is there a any simple solution for the common user:group pattern?

[faho]

Aaaah, okay, both of these provide a description (after a tab character), so only the users part is actually used (because the rest is now after a tab).

Here's what the actual chown completions do:

complete -c chown -d Username -a "(__fish_print_users):"
complete -c chown -d Username -a "(string match -r -- '.*:' (commandline -ct))(__fish_complete_groups)"

You could probably make a helper function that

1. if you don't have a ":" in the current token (commandline -ct) runs __fish_complete_users
2. If you do, prints the current token plus everything in __fish_complete_groups

(this would then also be useful in chown, and possibly elsewhere)

Or alternatively we can merge this as it stands and do that bit later.

[Jakeler]

Okay, I think it is best to merge as it is and make such a helper function in a separate PR.

[krobelus]

Since :somegroup is valid, that might need a separate entry (untested):
(echo; __fish_print_users):(__fish_complete_groups)

[faho]

Merged, thanks!