diff --check: minor fixups

There is no reason --exit-code and --check-diff must be mutually
exclusive, so assign different bits to different results and allow them
to be returned from the command.  Introduce diff_result_code() to factor
out the common code to decide final status code based on diffopt
settings and use it everywhere.

Update tests to match the above fix.

Turning pager off when "diff --check" is used is a regression.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-diff-index.c

@@ -44,9 +44,5 @@ int cmd_diff_index(int argc, const char **argv, const char *prefix)
 		return -1;
 	}
 	result = run_diff_index(&rev, cached);
-	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;
+	return diff_result_code(&rev.diffopt, result);
 }