John Li <jli@circularly.org> [Wed, 22 Aug 2012 11:18:35 -0400] rev 17386
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.