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>

shared.c

@@ -250,8 +250,8 @@ int cgit_refs_cb(const char *refname, const struct object_id *oid, int flags,
 	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;