amend: adjust the dirstate within a `parentchange` context

The adjustment in the direct consequence of the amend and the associated parents
change.

Differential Revision: https://phab.mercurial-scm.org/D11032

mercurial/cmdutil.py

@@ -2967,6 +2967,7 b' def amend(ui, repo, old, extra, pats, op'
         newid = repo.commitctx(new)
         ms.reset()
 
+        with repo.dirstate.parentchange():
         # Reroute the working copy parent to the new changeset
         repo.setparents(newid, repo.nullid)