changegroup: make delta header struct formatters actual structs
Why we weren't using compiled Struct instances, I don't know. They
make code simpler. In theory they are faster. Although I don't
believe I was able to measure any meaningful change. That could be
because this code is often dominated by compression, deltafication,
and function call overhead.
Differential Revision: https://phab.mercurial-scm.org/D4078
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