match: remove support for excludes from matcher class The support is now provided by differencematcher() and still available via the match() function.

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.

match: override matchfn() the usual way in subdirmatcher

match: make matchfn a method on the class This makes it easier to override in subclasses, so they don't have to assign the attribute with a lambda.