compat: Handle case where opts are None for <= 6.5
fix for https://bz.mercurial-scm.org/show_bug.cgi?id=6865
encountered when doing a backout on a changeset in a topic mercurial version 6.5.1
fix for https://bz.mercurial-scm.org/show_bug.cgi?id=6865
encountered when doing a backout on a changeset in a topic mercurial version 6.5.1