match: write forceincludematcher using unionmatcher
The forceincludematcher is simply a unionmatcher of a includematcher
(matching paths recursively) with the given matcher. Since the
forceincludematcher is only used by sparse, move it there.
I don't have a good sparse repo setup to test performance impact on.
Differential Revision: https://phab.mercurial-scm.org/D57