revlog: move references to revlog.hash to inside the revlog class This will make it possible for subclasses to have different hashing schemes when appropriate. I anticipate using this in manifests. Note that there's still one client of mercurial.revlog.hash() outside of revlog: mercurial.context.memctx uses it to construct the file entries in an in-memory manifest. I don't think this will be a problem in the immediate future, so I've left it as-is.