shared: make cgit_diff_tree_cb public

This will allow us to use this nice wrapper function elsewhere, avoiding
dealing with the diff queue when we only need to inspect a filepair.

Signed-off-by: John Keeping <john@keeping.me.uk>
diff --git a/shared.c b/shared.c
index a83afcb..0431b59 100644
--- a/shared.c
+++ b/shared.c
@@ -250,8 +250,8 @@
 	return 0;
 }
 
-static void cgit_diff_tree_cb(struct diff_queue_struct *q,
-			      struct diff_options *options, void *data)
+void cgit_diff_tree_cb(struct diff_queue_struct *q,
+		       struct diff_options *options, void *data)
 {
 	int i;