fix: ignore-from-file relative to config file

[jellehelsen]

This fixes the problem (#673) that files mentioned in ignore-from-file can not be found when running in a subdirectory. Values in ignore-from-file are now treated as being relative to the config file.

[coveralls]

coverage: 99.827% (+0.002%) from 99.825%
when pulling 174a64d on jellehelsen:fix/relative-ignore-from-file
into e118296 on adrienverge:master.

[adrienverge]

Hello!

Depending on the outcome of #673 and what @ndrwnaguib thinks, this PR could be welcome and merged 👍

I haven't reviewed yet, but I can already ask:

• Can you please minimize all the diff that can be? E.g. don't change config to c; keep validate_rule_conf() independant (not in a class) and pass it config_dir if needed; etc.
• Make the commit clean and ready to review. For example I saw a self.maxDiff = None.
• Document this new behavior by updating documentation, for example inside a new note below this one: https://github.com/adrienverge/yamllint/blob/8513d9b/docs/configuration.rst?plain=1#L229
• Please keep single quotes 🙏
• Add test cases for at least:
    - an ignore-from-file: file when file is in the root dir but current dir is a sub dir (should find it)
    - an ignore-from-file: file when file is in a sub dir but current dir is this sub dir (should error)
    - an ignore-from-file: file when configuration is not passed via a file (found)
    - an ignore-from-file: file when configuration is not passed via a file (error)
    - other cases?

[jellehelsen]

Hi,

I think I've addressed all your remarks.

• validate_rule_conf() is back out of the class and get passed the config_dir with None as default value, restored corresponding test
• Tests added and existing tests cleaned.
• Quotes restored (I should not have run ruff format. Apologies)
• Added a line of documentation at the suggested location to indicate that the ignore-from-file path is relative to the config file.