"diff --check" should affect exit status

"diff --check" should affect exit status

"git diff" has a --check option that can be used to check for whitespace
problems but it only reported by printing warnings to the
console.

Now when the --check option is used we give a non-zero exit status,
making "git diff --check" nicer to use in scripts and hooks.

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>