setdiscovery: use revset for resolving DAG heads in a subset...
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
