match: compose 'set:' pattern as matcher
Baby step towards porting fileset to matcher composition.
We can't use the exactmatcher since it would provide a computed set as exact
paths. That's why we use the predicatematcher with fset.contains. This
will be cleaned up later.
The test change in test-glog.t means that the "set:copied()" pattern is no
longer be processed as a slow path. That's because the fset is empty. This
will also change in future patches.