Filter out output object from C hashes

[alexcrichton]

Logic was added at 95082ad#diff-df3da71b6676696e6164c88404fb58a9R112 to filter out the object file from hash calculations. It was discovered in #112, however, that using output_file was the wrong output file (it was a "pretty" version, not the actual output file on the command line), so the logic was removed in #112.

Should figure out how to add it back in! (and if it's still necessary)