revlog: register changelogv2 C implementation in parsers This allows Python code to make use of the C implementation of the changelogv2 base operations when the C extensions are enabled. The `format_version` values are now shared between the C and Python sides, avoiding an additional translation for the selection of the format version to use. Differential Revision: https://phab.mercurial-scm.org/D12179

[workspace]
members = ["hg-core", "hg-cpython", "rhg"]
exclude = ["chg", "hgcli"]