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.

changeset:   0:9426b370c206
tag:         tip
user:        My Name <myname@example.com>
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     commit-1