fileset: use decorator to mark a predicate as "status caller" Before this patch, predicates calling 'matchctx.status()' are listed up by immediate list value in 'getfileset()'. This prevents 3rd party extensions from adding specific predicate calling 'matchctx.status()'. This uses decorator to mark a predicate as "status caller". This can also localize changes for adding (or removing) a "status caller" predicate function in source code.