CLJX auto processes files unnecessarily

cljx in auto mode will reprocess all files on any file change. This really slows down cljsbuild and will also cause multiple cljsbuild runs/test-refresh runs when they start before cljx has completed. This pull request ensures that only the changed file is reprocessed.

Hopefully there was not a good reason why this was the case in the first place. I couldn't think of any.