fileset: add a 'status(...)' predicate to control evaluation context Same as 'revs', this predicate does not select files but switches the evaluation context. This allow to match file according arbitrary status call. We can now express the same query as 'hg status'. The API (two 'revsingle' class) have been picked instead of a single 'revs' revset for multiple reasons: * it is less confusing to express * it allow to express more query (eg: backward status, cross branch status)