mercurial/cmdutil.py

--- a/mercurial/cmdutil.py	Wed Aug 27 15:30:09 2014 +0200
+++ b/mercurial/cmdutil.py	Wed Aug 27 23:10:06 2014 +0900
@@ -628,6 +628,9 @@
                     p2 = repo[nullid]
             except error.RepoError:
                 p1, p2 = parents
+            if p2.node() == nullid:
+                ui.warn(_("warning: import the patch as a normal revision\n"
+                          "(use --exact to import the patch as a merge)\n"))
         else:
             p1, p2 = parents