Fix memory leaks on error paths in listall_branches and diff_to_index

Fix Repository.listall_branches() leaking the result list when
git_branch_iterator_new() fails.

Fix Tree.diff_to_index() leaking the _pointer object when lazy-loading
the tree fails.

Fixes #1480

Assisted-by: Kimi Code

Author: jdavid

CHANGELOG.md

@@ -21,6 +21,10 @@
   on allocation or per-item failure
   [#1479](https://github.com/libgit2/pygit2/issues/1479).
 
+- Fix memory leaks in `Repository.listall_branches()` and
+  `Tree.diff_to_index()` on early error paths
+  [#1480](https://github.com/libgit2/pygit2/issues/1480).
+
 
 # 1.20.0 (2026-08-08)
 

src/repository.c

@@ -1438,8 +1438,10 @@ Repository_listall_branches_impl(Repository *self, PyObject *args, PyObject *(*i
     if (list == NULL)
         return NULL;
 
-    if ((err = git_branch_iterator_new(&iter, self->repo, list_flags)) < 0)
+    if ((err = git_branch_iterator_new(&iter, self->repo, list_flags)) < 0) {
+        Py_DECREF(list);
         return Error_set(err);
+    }
 
     while ((err = git_branch_next(&ref, &type, iter)) == 0) {
         PyObject *py_branch_name = item_trans(git_reference_shorthand(ref));

src/tree.c

@@ -308,7 +308,7 @@ Tree_diff_to_index(Tree *self, PyObject *args, PyObject *kwds)
     index = *((git_index **) buffer); /* the "buffer" contains the pointer */
 
     /* Call git_diff_tree_to_index */
-    if (Object__load((Object*)self) == NULL) { return NULL; } // Lazy load
+    if (Object__load((Object*)self) == NULL) { goto error; } // Lazy load
 
     err = git_diff_tree_to_index(&diff, self->repo->repo, self->tree, index, &opts);
     Py_DECREF(py_idx_ptr);