diff --no-index: also imitate the exit status of diff(1)

diff sets the exit status to 0 when no changes were found, to 1
when changes were found, and 2 means error.

We imitate this to be able to use "git diff" in the test scripts.
(Actually, keeping in line with the rest of git, -1 is returned
on error, which corresponds to an exit status 255).

To find out if the diff is not empty, a member called
"found_changes" was introduced in struct diff_options, which is
set in builtin_diff() and fn_out_consume().

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

diff.h

@@ -75,6 +75,9 @@ struct diff_options {
 	int stat_width;
 	int stat_name_width;
 
+	/* this is set by diffcore for DIFF_FORMAT_PATCH */
+	int found_changes;
+
 	int nr_paths;
 	const char **paths;
 	int *pathlens;