py3: use native strings as keys into **opts in chistedit stable

py3: use native strings as keys into **opts in chistedit

Now you should be able to successfully confirm your histedit plan (at
least in the case I tried). Even continuing after conflicts and
finishing the histedit worked.

Differential Revision: https://phab.mercurial-scm.org/D7186

--- a/hgext/histedit.py	Thu Oct 31 15:00:49 2019 -0700
+++ b/hgext/histedit.py	Thu Oct 31 15:03:12 2019 -0700
@@ -1687,7 +1687,7 @@
             with repo.vfs(b'chistedit', b'w+') as fp:
                 for r in rules:
                     fp.write(r)
-                opts[b'commands'] = fp.name
+                opts['commands'] = fp.name
             return _texthistedit(ui, repo, *freeargs, **opts)
     except KeyboardInterrupt:
         pass