Clarified message when nothing to merge is seen.

Sometimes there's a non-tip head waiting to be merged, especially after
pulling in remote changes while there were local modifications in the working
directory which had to be committed before 'hg merge' can work.

mercurial/localrepo.py

@@ -1643,8 +1643,8 b' class localrepository(object):'
         linear_path = (pa == p1 or pa == p2)
 
         if allow and linear_path:
-            raise util.Abort(_("there is nothing to merge, "
-                               "just use 'hg update'"))
+            raise util.Abort(_("there is nothing to merge, just use "
+                               "'hg update' or look at 'hg heads'"))
         if allow and not forcemerge:
             if modified or added or removed:
                 raise util.Abort(_("outstanding uncommitted changes"))