Fix plugin completion results parsing for ShellCompDirectiveFilterFileExt

[laurazard]

Compose V2 (and other plugins eventually) provide the extensions they want the shell to filter the autocompletion of files for:

$ /usr/libexec/docker/cli-plugins/docker-compose __completeNoDesc -f ""
yaml
yml
:8
Completion ended with directive: ShellCompDirectiveFilterFileExt

The current script does not process these results correctly, and instead we just provide a y (common beginning between yaml and yml)

$ docker compose -f <TAB><TAB>
yaml
yml

which is not the desired outcome.

- What I did

Check if the completion flag is :8 (ShellCompDirectiveFilterFileExt) and format a valid glob pattern to feed into _filedir when this is the case

- How to verify it

Load the new completion script, and type

docker compose -f <TAB><TAB>

verify that completions are provided for files with the extension .yaml or .yml,

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Merging #4136 (4896123) into master (88924b1) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4136   +/-   ##
=======================================
  Coverage   59.16%   59.16%           
=======================================
  Files         287      287           
  Lines       24716    24716           
=======================================
  Hits        14623    14623           
  Misses       9209     9209           
  Partials      884      884           

[thaJeztah]

@tianon @albers ptal 🤗

[tianon]

If anyone else reviewing this was unfamiliar with what this means, it's a Cobra thing, and https://github.com/spf13/cobra/blob/4dd4b25de38418174a6e859e8a32eaccca32dccc/completions_test.go#L1046-L1047 is the best reference I could find for it being :8 (there's probably a better one, but that's what my search brought me to 😅).

[laurazard]

Thanks! I'd thought about adding some link explaining it better and then totally forgot :|

[thaJeztah]

I'll skip merging this one for now, to have a look if we want to add Tianon's suggestions (as it's marked for cherry-picking, it makes it a bit cleaner to do the backport "in one go" instead of a PR-and-follow-up).

[albers]

After finally realizing that the Completion ended with directive: ShellCompDirectiveFilterFileExt line actually goes to stderr: LGTM, thanks @laurazard

[thaJeztah]

Just checking up on this; Is the suggestion from @tianon (echo -> tail) something to consider, or "too minor to bother")? #4136 (comment)

[thaJeztah]

LGTM, thank you!