[RFC] diff: support custom callbacks for output

---
I'm re-writing git-status in C, and that entails using the git library
to replace calls to things like git-diff-index. Fortunately,
run_diff_index does just what I want. Unfortunately, it ends up wanting
to send its output to stdout in one of the pre-defined diff formats, and
what I really want is my own status format.

I could, of course, add diff_flush_status() support to diffcore.
However, I'm not sure that it really belongs there. In the interests of
libification, it seems reasonable for diffcore to provide a callback
mechanism for each filepair.

Below is an attempt to provide such a mechanism. My questions are:
  1. Am I right that there is no other way to accomplish this task? This
     is my first interaction with diffcore, so I might have overlooked
     something.
  2. Is this the way I should go about it, or should I add a status
     output to git-diff? (Note that the callback takes the same
     parameters as the other diff_flush_* functions; it may be that we
     can use this general mechanism to implement other formats).
  3. This seems very similar in spirit to the add_remove and changed
     members of diff_options. However, I'm not clear on when those are
     called (they seem to only be called from diff-tree).

 diff.c |    8 ++++++++
 diff.h |    7 +++++++
 2 files changed, 15 insertions(+), 0 deletions(-)

diff --git a/diff.c b/diff.c
index f5c9a55..a04257f 100644
--- a/diff.c
+++ b/diff.c

@@ -2278,6 +2278,14 @@ void diff_flush(struct diff_options *opt
 		}
 	}
 
+	if (output_format & DIFF_FORMAT_CALLBACK) {
+		for (i = 0; i < q->nr; i++) {
+			struct diff_filepair *p = q->queue[i];
+			if (check_pair_status(p))
+				options->format_callback(p, options);
+		}
+	}
+
 	for (i = 0; i < q->nr; i++)
 		diff_free_filepair(q->queue[i]);
 free_queue:

diff --git a/diff.h b/diff.h
index 2cced53..5671a53 100644
--- a/diff.h
+++ b/diff.h

@@ -8,6 +8,7 @@ #include "tree-walk.h"
 
 struct rev_info;
 struct diff_options;
+struct diff_filepair;
 
 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 d
 		    const unsigned char *sha1,
 		    const char *base, const char *path);
 
+typedef void (*diff_format_fn_t)(struct diff_filepair *p,
+		struct diff_options *options);
+
 #define DIFF_FORMAT_RAW		0x0001
 #define DIFF_FORMAT_DIFFSTAT	0x0002
 #define DIFF_FORMAT_SUMMARY	0x0004

@@ -35,6 +39,8 @@ #define DIFF_FORMAT_CHECKDIFF	0x0040
  */
 #define DIFF_FORMAT_NO_OUTPUT	0x0080
 
+#define DIFF_FORMAT_CALLBACK	0x0100
+
 struct diff_options {
 	const char *filter;
 	const char *orderfile;

@@ -67,6 +73,7 @@ struct diff_options {
 	int *pathlens;
 	change_fn_t change;
 	add_remove_fn_t add_remove;
+	diff_format_fn_t format_callback;
 };
 
 enum color_diff {

-- 
1.4.2.rc3.g539fa-dirty