[libgit2-glib] diff: it is an error to pass NULL for both the `old_tree` and `new_tree`.

[Ignacio Casal Quinteiro <icq src gnome org>]

commit 204fc976f8d2d2f227335ce4d67ee26b711a5c5a
Author: Ignacio Casal Quinteiro <icq gnome org>
Date:   Thu Jun 20 09:36:06 2013 +0200

    diff: it is an error to pass NULL for both the `old_tree` and `new_tree`.

 libgit2-glib/ggit-diff.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/libgit2-glib/ggit-diff.c b/libgit2-glib/ggit-diff.c
index 624c191..583ee37 100644
--- a/libgit2-glib/ggit-diff.c
+++ b/libgit2-glib/ggit-diff.c
@@ -164,6 +164,7 @@ ggit_diff_new_tree_to_tree (GgitRepository   *repository,
        g_return_val_if_fail (GGIT_IS_REPOSITORY (repository), NULL);
        g_return_val_if_fail (old_tree == NULL || GGIT_IS_TREE (old_tree), NULL);
        g_return_val_if_fail (new_tree == NULL || GGIT_IS_TREE (new_tree), NULL);
+       g_return_val_if_fail (new_tree != NULL || new_tree != NULL, NULL);
        g_return_val_if_fail (error == NULL || *error == NULL, NULL);
 
        ret = git_diff_tree_to_tree (&diff,