revlog: split the content verification of a node into a separate method This will be used by LFS to tune what is skipped. In the future, this could also be used by LFS to indicate which nodes tagged with `skipread` are simply in need of a blob fetch, so that they can be done in a batch later. (Currently, `skipread` also indicates censored data and errors.) Additionally, it could be used to cache the sha1 hash value for each blob so that large blobs don't need to be re-read and re-hashed if they are used by multiple nodes. Differential Revision: https://phab.mercurial-scm.org/D7710