diff: drop useless return values in git-diff helpers

diff: drop useless return values in git-diff helpers

Since git-diff has many diff modes, it dispatches to many helpers to
perform each one. But every helper simply returns "0", as it exits
directly if there are serious errors (and options like --exit-code are
handled afterwards). So let's get rid of these useless return values,
which makes the code flow more clear.

There's very little chance that we'd later want to propagate errors
instead of dying immediately. These are all static-local helpers for the
git-diff program implementing its various modes. More "lib-ified" code
would directly call the underlying functions.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>