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

# See http://EditorConfig.org for the specification

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