revset: use decorator to register a function as revset predicate Using decorator can localize changes for adding (or removing) a revset predicate function in source code. It is also useful to pick predicates up for specific purpose. For example, subsequent patch marks predicates as "safe" by decorator. This patch defines 'parsefuncdecl()' in 'funcregistrar' class, because this implementation can be uesd by other decorator class for fileset predicate and template function.