backout: use cmdutil.revert directly instead of commands.revert

Before this change, backout would explicitly set the options it passed
to commands.revert in order to fall thru most of its logic and call
cmdutil.revert. This change makes it clearer what backup is trying to
accomplish and makes it robust against changes to the revert command.

mercurial/commands.py

@@ -458,12 +458,8 b' def backout(ui, repo, node=None, rev=Non'
         branch = repo.dirstate.branch()
         hg.clean(repo, node, show_stats=False)
         repo.dirstate.setbranch(branch)
-        revert_opts = opts.copy()
-        revert_opts['date'] = None
-        revert_opts['all'] = True
-        revert_opts['rev'] = hex(parent)
-        revert_opts['no_backup'] = None
-        revert(ui, repo, **revert_opts)
+        rctx = scmutil.revsingle(repo, hex(parent))
+        cmdutil.revert(ui, repo, rctx, repo.dirstate.parents())
         if not opts.get('merge') and op1 != node:
             try:
                 ui.setconfig('ui', 'forcemerge', opts.get('tool', ''))