Show informative messages when there is nothing to checkout:
- checkout_file: "Nothing to checkout." when no modified files
- checkout_tag: "Nothing to checkout: there are no tags."
This is a breaking change: flag arguments provided to forgit clean are
now passed to git ls-files instead of git clean -n. This change is
necessary because git clean does not have a -z option that would prevent
it from escaping file names containing backslashes. However, git ls-files
is way more flexible than git clean, so all use cases covered by git
clean (and more) should still be possible.
