print message after backout that tells that backout adds new head.

mercurial/commands.py

@@ -836,9 +836,14 b' def backout(ui, repo, rev, **opts):'
         return '%d:%s' % (repo.changelog.rev(node), short(node))
     ui.status(_('changeset %s backs out changeset %s\n') %
               (nice(repo.changelog.tip()), nice(node)))
-    if opts['merge'] and op1 != node:
-        ui.status(_('merging with changeset %s\n') % nice(op1))
-        doupdate(ui, repo, hex(op1), **opts)
+    if op1 != node:
+        if opts['merge']:
+            ui.status(_('merging with changeset %s\n') % nice(op1))
+            doupdate(ui, repo, hex(op1), **opts)
+        else:
+            ui.status(_('the backout changeset is a new head - '
+                        'do not forget to merge\n'))
+            ui.status(_('(use "backout -m" if you want to auto-merge)\n'))
 
 def bundle(ui, repo, fname, dest="default-push", **opts):
     """create a changegroup file

tests/test-backout.out

@@ -26,4 +26,6 b' adding a'
 adding b
 reverting a
 changeset 3:4cbb1e70196a backs out changeset 1:22bca4c721e5
-b
+the backout changeset is a new head - do not forget to merge
+(use "backout -m" if you want to auto-merge)
+b: No such file or directory