Distributed project.ignore

[dmezh]

Today, you can tell buck2 to ignore files using ignore = in [project] in .buckconfig. Maybe we could support something like a .buckignore file (semantically equivalent to .gitignore/.hgignore)? If we could specify a spec for this filename in buckconfig and adopt .git/hgignore syntax, we could even reuse existing .git/hgignore files.

This differs from .bazelignore which is still only one file at the root (afaict).