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

mercurial/cmdutil.py

@@ -3039,7 +3039,6 b' def amend(ui, repo, commitfunc, old, ext'
     ui.note(_('amending changeset %s\n') % old)
     base = old.p1()
 
-    newid = None
     with repo.wlock(), repo.lock(), repo.transaction('amend'):
         # See if we got a message from -m or -l, if not, open the editor
         # with the message of the changeset to amend
@@ -3164,6 +3163,7 b' def amend(ui, repo, commitfunc, old, ext'
             return old.node()
 
         ph = repo.ui.config('phases', 'new-commit', phases.draft)
+        newid = None
         try:
             if opts.get('secret'):
                 commitphase = 'secret'