combine-diff: refactor built-in xdiff interface.

This refactors the line-by-line callback mechanism used in
combine-diff so that other programs can reuse it more easily.

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

xdiff-interface.h

@@ -0,0 +1,18 @@
+#ifndef XDIFF_INTERFACE_H
+#define XDIFF_INTERFACE_H
+
+#include "xdiff/xdiff.h"
+
+struct xdiff_emit_state;
+
+typedef void (*xdiff_emit_consume_fn)(void *, char *, unsigned long);
+
+struct xdiff_emit_state {
+	xdiff_emit_consume_fn consume;
+	char *remainder;
+	unsigned long remainder_size;
+};
+
+int xdiff_outf(void *priv_, mmbuffer_t *mb, int nbuf);
+
+#endif