amend: moving first assignment of newid closer to its use newid was needlessly further away from where its intended to be used leading to bad readability. This commit moves it to address the same. The end goal is to remove the redundant commit in the amend code path and this commit takes care of cleaning up some unrelated code before that change. Test Plan: ran the test suite Differential Revision: https://phab.mercurial-scm.org/D597

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