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
Greg Ward, author of the original bfiles extension
Na'Tosha Bard of Unity Technologies
Fog Creek Software
Special thanks to the University of Toronto and the UCOSP program