match: handle excludes using new differencematcher As I've said on earlier patches, I'm hoping to use more composition of simpler matchers instead of the single complex matcher we currently have. This extracts a first new matcher that composes two other matchers. It matches if the first matcher matches but the second does not. As such, we can use it for excludes, which this patch also does. We'll remove the now-unncessary code for excludes in the next patch.