changegroup: refactor delta parent code We had recently abstracted the delta parent functions to facilitate extracting code from cgpacker. Now that we're in a better place, it is time to revisit the design. Changegroup version 1 requires that the previous node be used as the delta parent. Later versions allow any available node to be used as the base. In the case where an arbitrary parent can be used, the choice of a delta parent is best left in the hands of the storage backend. So it makes sense for the delta parent selection to be hidden away in the storage layer. This means deferring the choice of the delta parent selection function to as close to delta generation time as possible. This commit moves the delta selection logic to essentially just before delta generation. However, because changegroup version 1 limits what we can do, we have retained the ability to force a delta against the previous revision. As part of this, I realized that the ellipsis parent function was unused! That's because ellipsis mode always sends full revisions and not deltas. Differential Revision: https://phab.mercurial-scm.org/D4214