py33: sort the files parameters of "--files"

In Python 3.3, hash randomization is enabled by default. It causes the
iteration order of dicts and sets to be unpredictable and differ across
Python runs.

Sort the files parameters when multiple files are designated.

Close-Bug #1235096

Change-Id: I0db645b3fb92408bd8c6780bfabe7ce8265a65a8