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
running: init test1
result: 0
running: add foo
result: 0
running: commit -m commit1 -d 2000-01-01 foo
result: 0
running: commit -m commit2 -d 2000-01-02 foo
result: 0
running: log -r 0
changeset: 0:0e4634943879
user: test
date: Sat Jan 01 00:00:00 2000 +0000
summary: commit1
result: 0
running: log -r tip
changeset: 1:45589e459b2e
tag: tip
user: test
date: Sun Jan 02 00:00:00 2000 +0000
summary: commit2
result: 0