Ensure cljx auto only reprocesses changed files

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.