Add --gitignore option

[azu]

Summary

Secretlint has used globby and globby has gitignore option.

It is useful that user can use secretlint --gitignore "**/*" instead of secretlint --secretlintignore .gitignore "**/*".

Basic example

If use want to respect gitinore, --gitignore enable this feature.

# respect .secretlintignore and .gitignore
$ secretlint --gitignore  "**/*"

This feature should be opt-in, as some secrets may already be included under git.

Ref #204 (comment)

Motivation

secretlint --secretlintignore .gitignore is not intuitive.
Also, secretlint --secretlintignore .gitignore disable .secretlintignore
secretlint --gitignore respects .gitignore and .secretlintignore.

Additionally, gitignore option will cover edge case. It is more safe.

Naming

We need to think easy to understand name.

• --gitignore: Respect .gitignore file

Another option:

globby has ignoreFiles option.

Memo

I do not known if the gitignore option respects cascaded .gitignore files.

[azu]

📝 I received a message that current secretlint --secretlintignore .gitignore is something wrong.
https://twitter.com/takeokunn/status/1581667659329855488
This feature may resolve it.