"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>

builtin-diff-files.c

@@ -33,5 +33,7 @@ int cmd_diff_files(int argc, const char **argv, const char *prefix)
 	result = run_diff_files_cmd(&rev, argc, argv);
 	if (DIFF_OPT_TST(&rev.diffopt, EXIT_WITH_STATUS))
 		return DIFF_OPT_TST(&rev.diffopt, HAS_CHANGES) != 0;
+	if (rev.diffopt.output_format & DIFF_FORMAT_CHECKDIFF)
+		return DIFF_OPT_TST(&rev.diffopt, CHECK_FAILED) != 0;
 	return result;
 }