diff: add diff_line_prefix function

This is a helper function to call the diff output_prefix function and
return its value as a C string, allowing us to greatly simplify
everywhere that needs to get the output prefix.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -1105,6 +1105,16 @@ const char *diff_get_color(int diff_use_color, enum color_diff ix)
 	return "";
 }
 
+const char *diff_line_prefix(struct diff_options *opt)
+{
+	struct strbuf *msgbuf;
+	if (!opt->output_prefix)
+		return "";
+
+	msgbuf = opt->output_prefix(opt, opt->output_prefix_data);
+	return msgbuf->buf;
+}
+
 static unsigned long sane_truncate_line(struct emit_callback *ecb, char *line, unsigned long len)
 {
 	const char *cp;

diff.h

@@ -174,6 +174,9 @@ const char *diff_get_color(int diff_use_color, enum color_diff ix);
 	diff_get_color((o)->use_color, ix)
 
 
+const char *diff_line_prefix(struct diff_options *);
+
+
 extern const char mime_boundary_leader[];
 
 extern void diff_tree_setup_paths(const char **paths, struct diff_options *);