branchcache: have a hasnode function to validate nodes Upcoming patches will delay node validation until it's required. So we need to a way in branchcache class to check whether a node exists or node. Other options were making repo or changelog an attribute of branchcache object. But the branchcache depends on filters so I decided to pass a function object. Differential Revision: https://phab.mercurial-scm.org/D6157