changegroup: use changelogrevision()
Using offsets for accessing changelog entries isn't very readable.
As a bonus, changelog.changelogrevision() also accepts a revision,
so we don't need to perform the inline node resolution either.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL