Don't overwrite cffi files if they haven't changed (#5553)
Don't overwrite cffi files if they haven't changed (#5553)

I've seen cargo doing more work than it should as a result of these
files being rewritten.
wisechengyi/pants,tdyas/pants,UnrememberMe/pants,baroquebobcat/pants,wisechengyi/pants,benjyw/pants,benjyw/pants,wisechengyi/pants,UnrememberMe/pants,wisechengyi/pants,baroquebobcat/pants,baroquebobcat/pants,pantsbuild/pants,twitter/pants,twitter/pants,twitter/pants,wisechengyi/pants,pantsbuild/pants,benjyw/pants,foursquare/pants,baroquebobcat/pants,UnrememberMe/pants,twitter/pants,wisechengyi/pants,foursquare/pants,baroquebobcat/pants,UnrememberMe/pants,tdyas/pants,UnrememberMe/pants,pantsbuild/pants,baroquebobcat/pants,twitter/pants,twitter/pants,pantsbuild/pants,wisechengyi/pants,tdyas/pants,foursquare/pants,jsirois/pants,UnrememberMe/pants,baroquebobcat/pants,foursquare/pants,wisechengyi/pants,pantsbuild/pants,pantsbuild/pants,foursquare/pants,tdyas/pants,jsirois/pants,tdyas/pants,foursquare/pants,UnrememberMe/pants,pantsbuild/pants,benjyw/pants,twitter/pants,tdyas/pants,benjyw/pants,baroquebobcat/pants,UnrememberMe/pants,jsirois/pants,twitter/pants,twitter/pants,UnrememberMe/pants,tdyas/pants,foursquare/pants,benjyw/pants,foursquare/pants,wisechengyi/pants,foursquare/pants,baroquebobcat/pants,tdyas/pants,tdyas/pants,benjyw/pants