Add --git-ignore-pattern option.

[inakimalerba]

It can be used to ignore certain paths or files from the repo.is_clean check. It's an alternative to the --git-ignore-new parameter, when you only need to skip a certain path.

This comes from the need to store ccache inside the working directory on Gitlab pipelines.

Issue: https://salsa.debian.org/salsa-ci-team/pipeline/issues/27 Workaround: https://salsa.debian.org/salsa-ci-team/pipeline/merge_requests/63/diffs#d02988742a4e38af0efe3a7f4c522afc7e77728a_34_41

[agx]

Thanks for the patch! Some improvements so we can merge this:

• [ ] add a test for the behaviour
• [ ] rename to --git-ignore-new-pattern (to match --git-ignore-new)
• [ ] allow other commands that have --git-ignore-new to use it as well
• [ ] update the docs
• [ ] allow the command multiple times so