Don't try to merge if there is nothing to merge

Compare the version of the downstream and upstream branches before trying to merge, such that a merge can be skipped if upstream is newer.

While there also change the argument names to branch names rather then suites to make the tool a bit more flexible (in case upstream isn't debian and downstream isn't apertis.

Signed-off-by: Sjoerd Simons sjoerd.simons@collabora.co.uk