merge: handle case when heads are all bookmarks

If all heads are bookmarks, merge fails to find what node to merge
with (throws an IndexError while indexing into the non-bookmark heads
list) as of 4a02cf4fbb2e. This catches that case and prints an error
to specify a rev explicitly.

mercurial/commands.py

@@ -4269,7 +4269,7 b' def merge(ui, repo, node=None, **opts):'
                              hint=_("run 'hg heads .' to see heads"))
 
         parent = repo.dirstate.p1()
-        if len(nbhs) == 1:
+        if len(nbhs) <= 1:
             if len(bheads) > 1:
                 raise util.Abort(_("heads are bookmarked - "
                                    "please merge with an explicit rev"),