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
Hello, Foobar
[b'Hello, Fo, =;:<o', b'Hello, Bar']