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.

Is there an error message when trying to diff non-existing files?
found: No such file or directory
not: No such file or directory
Is there an error message when trying to add non-existing files?
found: No such file or directory
not: No such file or directory