dagutil: remove unused classes We only directly use revlogdag in changegroup code. We don't need all this abstraction. So remove various classes and levels of inheritance. Differential Revision: https://phab.mercurial-scm.org/D4325

setdiscovery: use revset for resolving DAG heads in a subset This was the final use of dagutil in setdiscovery! For reasons I didn't investigate, feeding a set with nullrev into the heads() revset resulted in a bunch of tests failing. Filtering out nullrev from the input set fixes things. Differential Revision: https://phab.mercurial-scm.org/D4324