tests/helper-runtests.sh

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

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL