fileset: use decorator to mark a function as fileset predicate Using decorator can localize changes for adding (or removing) a fileset predicate function in source code. It is also useful to pick predicates up for specific purpose. For example, subsequent patches marks predicates as "call status" or "use existing" via decorator. To avoid (1) redundancy between "predicate name" and (the beginning of) help document, and (2) accidental typo of help document, this patch also makes decorator put predicate declration into the beginning of help.