Force-include is a misnomer because it won't include files
that weren't considered at all (like files in an excluded subdir).
Instead, call it "unexclude" to make it slightly clearer that this
will just override the exclusions.
Support multiple roots in config file, not just one.
The absolute path stuff before would match against exclusions/inclusions
based on paths from the root dir, but that doesn't make sense when we
have multiple roots, and added needless complexity.