Refactor mel.cmd.rotomapdiff: specialize on 1 map
Refactor mel.cmd.rotomapdiff: specialize on 1 map

Remove the generalization to printing multiple diffs, this allows us to
change a list to a single item and thereby remove loops and other
complications.

Test Plan:
$ ./meta/run_tests.sh
.. 'mel rotomap-diff' on a rotomap ..

aevri/mel,aevri/mel