Handle linebreaks in custom completions.

[Luap99]

If a command/flag description contains a linebreak than the shell completion script will interpret this as new command/flag.

To fix this we should only use the first line from the description in the output.

e.g. cmd.Flags().String("flag", "", "Description for flag\nlonger description")

[CLAassistant]

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[Luap99]

@marckhouzam Please take a look.
I noticed this while testing #1146 because this also messed up the formatting of the bash descriptions.

[marckhouzam]

@Luap99 Interesting find! What do you think would be best for a user:

1. only taking the first line of the description or
2. escaping the newline and showing the entire description (if we can)

I'm asking because I wondered if truncating a description would remove useful information.
Do you have concrete examples of descriptions using a newline so we can judge what would be best?

[Luap99]

@marckhouzam Here are two examples I found in the podman project:
tmpdir flag
latest flag

I would argue that the first line should contain enough information to understand what a command/flag does. If someone wants the full information they should use the man pages or --help.

[marckhouzam]

I tested this and it works great. Just a couple of nits on the comments.
Thanks!

[marckhouzam]

LGTM
This will protect both fish and zsh completions from descriptions that have a line break in them.

/cc @jharshman

[jpmcb]

Thanks for this fix! LGTM 🎸