py3: avoid another b''.format() in chistedit stable

py3: avoid another b''.format() in chistedit

Now you'll get a different traceback if you try to confirm you
histedit plan.

Differential Revision: https://phab.mercurial-scm.org/D7184

--- a/hgext/histedit.py	Thu Oct 31 15:02:03 2019 -0700
+++ b/hgext/histedit.py	Thu Oct 31 15:02:48 2019 -0700
@@ -1322,7 +1322,7 @@
     our list of rules"""
     commands = []
     for rules in rules:
-        commands.append(b"{0} {1}\n".format(rules.action, rules.ctx))
+        commands.append(b'%s %s\n' % (rules.action, rules.ctx))
     return commands