index: add failing test for writing an invalid tree from an unowned index

When the index does not belong to any repository, we do not do any checks of the
target id going in as we cannot verify that it exists.

When we then write it out to a repository as a tree, we fail to perform the
object existance and type-matching check that we do in other code-paths. This
leads to being able to write trees which point to non-existent blobs even with
strict object creation enabled.

tests/index/tests.c

@@ -639,6 +639,30 @@ static void write_invalid_filename(git_repository *repo, const char *fn_orig)
 	git__free(fn);
 }
 
+void test_index_tests__write_tree_invalid_unowned_index(void)
+{
+	git_index *idx;
+	git_repository *repo;
+	git_index_entry entry = {{0}};
+	git_oid tree_id;
+
+	cl_git_pass(git_index_new(&idx));
+
+	cl_git_pass(git_oid_fromstr(&entry.id, "8312e0a89a9cbab77c732b6bc39b51a783e3a318"));
+	entry.path = "foo";
+	entry.mode = GIT_FILEMODE_BLOB;
+	cl_git_pass(git_index_add(idx, &entry));
+
+	cl_git_pass(git_repository_init(&repo, "./invalid-id", 0));
+
+	cl_git_fail(git_index_write_tree_to(&tree_id, idx, repo));
+
+	git_index_free(idx);
+	git_repository_free(repo);
+
+	cl_fixture_cleanup("invalid-id");
+}
+
 /* Test that writing an invalid filename fails */
 void test_index_tests__write_invalid_filename(void)
 {