fileset: add "tracked()" to explicitly select files in the revision I'm going to rewrite filesets to be match predicates, which means basic patterns such as '*' will no longer be "closed" to the subset constructed from the ctx. Good thing is that 'hg status "set:not binary()"' can include unknown files out of the box, and fileset computation will likely to be faster as we won't have to walk dirstate twice, for example. Bad thing is that we can't select files at a certain revision by 'set:revs(REV, **)' since '**' is "open" to any paths. So, this patch introduces "tracked()" as a replacement for the '**' in the example above.