diff: support custom callbacks for output

Users can request the DIFF_FORMAT_CALLBACK output format to get a callback
consisting of the whole diff_queue_struct.

Signed-off-by: Junio C Hamano <junkio@cox.net>

diff.c

@@ -2587,6 +2587,9 @@ void diff_flush(struct diff_options *options)
 		}
 	}
 
+	if (output_format & DIFF_FORMAT_CALLBACK)
+		options->format_callback(q, options, options->format_callback_data);
+
 	for (i = 0; i < q->nr; i++)
 		diff_free_filepair(q->queue[i]);
 free_queue:

diff.h

@@ -8,6 +8,7 @@
 
 struct rev_info;
 struct diff_options;
+struct diff_queue_struct;
 
 typedef void (*change_fn_t)(struct diff_options *options,
 		 unsigned old_mode, unsigned new_mode,
@@ -20,6 +21,9 @@ typedef void (*add_remove_fn_t)(struct diff_options *options,
 		    const unsigned char *sha1,
 		    const char *base, const char *path);
 
+typedef void (*diff_format_fn_t)(struct diff_queue_struct *q,
+		struct diff_options *options, void *data);
+
 #define DIFF_FORMAT_RAW		0x0001
 #define DIFF_FORMAT_DIFFSTAT	0x0002
 #define DIFF_FORMAT_SUMMARY	0x0004
@@ -35,6 +39,8 @@ typedef void (*add_remove_fn_t)(struct diff_options *options,
  */
 #define DIFF_FORMAT_NO_OUTPUT	0x0080
 
+#define DIFF_FORMAT_CALLBACK	0x0100
+
 struct diff_options {
 	const char *filter;
 	const char *orderfile;
@@ -68,6 +74,8 @@ struct diff_options {
 	int *pathlens;
 	change_fn_t change;
 	add_remove_fn_t add_remove;
+	diff_format_fn_t format_callback;
+	void *format_callback_data;
 };
 
 enum color_diff {