Include --help and --version flag in completion

[marckhouzam]

Fixes #1786

Thanks to @fnickels's nice work on #1707, I realized why the --help/-h and --version/-v flags where not included in shell completions.

The --help, -h, --version and -v flags are normally added when the execute() function is called on a command. When doing completion we don't call execute() so we need to add these flags explicitly to the command being completed.

Also, when doing shell completion following those flags, this PR disables all completions.
To do so, we keep track of if the --help and --version flags were added by Cobra using a new flag annotation.

To test:

./prog -<TAB>
# Notice that --help and -h are listed with this PR but not without it

./prog --help <TAB>
# Nothing is suggested with this PR but file completion is triggered without the PR

[marckhouzam]

I realized that the completion logic for this improvement is still lacking. For example: helm status --help <TAB> will still suggest a list of helm releases.

I will need to treat the --help and --version flags as exceptions in Cobra. This actually make sense since these flags are added by Cobra. (Bear with my being a bit OCD about making completion work perfectly 😉 )

I will post an updated version soon.

[jpmcb]

Nice! This all sounds good - give me a ping once this is ready to review! :D

[github-actions]

This PR exceeds the recommended size of 200 lines. Please make sure you are NOT addressing multiple issues with one PR. Note this PR might be rejected due to its size.

[github-actions]

This PR exceeds the recommended size of 200 lines. Please make sure you are NOT addressing multiple issues with one PR. Note this PR might be rejected due to its size.

[marckhouzam]

Alright, this is ready @jpmcb. If you ignore the 234 lines of new tests, the change is fairly straightforward (< 30 lines).

One important aspect that you may have a different opinion on is that I have added a new flag annotation: FlagSetByCobraAnnotation. It is used to keep track if a flag was added by Cobra (the help and version flags). This was to avoid short-circuiting completions if these flags were program flags (not added by Cobra); if it is the program that adds such flags, it is the program's responsibility to handle shell completion adequately.

What to look for when testing (any generic test program will do):

1. see the --help and -h flags being completed for any command (e.g., prog -<TAB>)
2. see the --version and -v flags being completed for commands that have their Version field set (e.g., prog -<TAB>)
3. no further completion AFTER any of --help, -h, --version and -v, but only if they were created by Cobra (not created by the program itself). Note that there are also Go tests added for this case. (e.g., prog -h <TAB>)

Also the tests of the cobra-completion-testing repo all pass.
I have also done some manual testing with helm.