test-editor-filename: fix portability of fake editor command - /bin/bash doesn't exist on FreeBSD - edit is executed by cmd.exe on Windows

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