log: rewrite --follow-first -rREV like --follow for consistency (BC)
This helps fixing the "--follow -rREV PATH" issue.
.. bc::
``log --follow-first -rREV``, which is deprecated, now follows the first
parent of merge revisions from the specified ``REV`` just like
``log --follow -rREV``.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}