Fix broken pipe error, issue #192

[GunterSchmidt]

Fixes #192 with code used in coreutils.

[GunterSchmidt]

This fix will return

>diffutils diff /etc/pacman.conf /dev/null | :
diffutils: write error: Broken pipe

while GNU diff does suppresses it completely.

This could easily be changed, but I prefer to output this error message.

Reason: Using diff on different files should return exit code 1, but returns in this case 0 without an error message. This is completely wrong information to the user.

This version sets exit code 1, which seems correct, since |: would not produce an error if no output was written as in the equal case.