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
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.