Remove OrderedSet from merge_files()
Remove OrderedSet from merge_files()

I just can't remember why this is here. It was on purpose though.

ViDA-NYU/reprozip,JosuaKrause/reprozip,JosuaKrause/reprozip,ViDA-NYU/reprozip,ViDA-NYU/reprozip,ViDA-NYU/reprozip,JosuaKrause/reprozip,JosuaKrause/reprozip,ViDA-NYU/reprozip