amend: rectify comment
Comment was ambiguous as there can be two parents of a changeset in mercurial.
This commit fixes the comment to clarify that the first parent is being
considered.
Test Plan:
ran the test suite
Differential Revision: https://phab.mercurial-scm.org/D595
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}