dagop: extract headsetofconnecteds() from dagutil The functionality for resolving the set of DAG heads from a subset simply requires a function to resolve parent revisions. Let's establish a function in the dagop module to do this, which seems to be where generic DAG functionality goes these days. Differential Revision: https://phab.mercurial-scm.org/D4327

setdiscovery: precompute children revisions to avoid quadratic lookup Moving away from dagutil a few commits ago introduced quadratic behavior when resolving children revisions during discovery. This commit introduces a precompute step of the children revisions to avoid the bad behavior. I believe the new code should have near identical performance to what dagutil was doing before. Behavior is still slightly different because we take into account filtered revisions. But this change was made when we moved off dagutil. I added a comment about multiple invocations of this function redundantly calculating the children revisions. I believe this potentially undesirable behavior was present when we used dagutil, as the call to inverse() previously in this function created a new object and required computing children on every invocation. I thought we should document the potential for a performance issue rather than let it go undocumented. Differential Revision: https://phab.mercurial-scm.org/D4326

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