bundle: introduce per-engine compression level
If experimental.bundlecomplevel.$engine is set, prefer it over the
generic experimental.bundlecomplevel. Given that compression levels have
widely different meanings across engines, this allows much saner
configuration.
Differential Revision: https://phab.mercurial-scm.org/D3393
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.