changegroup: control delta parent behavior via constructor
The last remaining override on cg2packer related to parent delta
computation. We pass a parameter to the constructor to control
whether to delta against the previous revision and we inline all
parent delta logic into a single function.
With this change, cg2packer is empty, so it has been deleted.
Differential Revision: https://phab.mercurial-scm.org/D4083
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