fileset: restrict getfileset() to not return a computed set (API) And rename the functions accordingly. fileset.match() will be changed to not compute the initial subset. test-glog*.t get back to the state before 9f9ffe5f687c "match: compose 'set:' pattern as matcher."

match: add prefixdirmatcher to adapt subrepo matcher back This serves as an inverse function to the subdirmatcher, and will be used to wrap a fileset matcher of subrepositories. One of the root/prefix paths could be deduced from the matcher attributes to be wrapped, but we don't since the callers of this class know the root/prefix paths and can simply pass them in.

fileset: make debugfileset filter repository files This prepares for the structural change of the fileset. A computed fileset will no longer be a set of files, but a boolean function (i.e. matcher) to test if an input file matches the given fileset expression. --all-files option is added because some examples in the test need to scan files across revisions.