merge_trees: introduce test for submodule renames

Test that shows that submodules are incorrectly considered in renames,
and `git_merge_trees` will fail to lookup the submodule as a blob.

tests/merge/trees/renames.c

@@ -252,3 +252,25 @@ void test_merge_trees_renames__no_rename_index(void)
 
 	git_index_free(index);
 }
+
+void test_merge_trees_renames__submodules(void)
+{
+	git_index *index;
+	git_merge_options *opts = NULL;
+
+	struct merge_index_entry merge_index_entries[] = {
+		{ 0100644, "cd3e8d4aa06bdc781f264171030bc28f2b370fee", 0, ".gitmodules" },
+		{ 0100644, "4dd1ef7569b18d92d93c0a35bb6b93049137b355", 1, "file.txt" },
+		{ 0100644, "a2d8d1824c68541cca94ffb90f79291eba495921", 2, "file.txt" },
+		{ 0100644, "63ec604d491161ddafdae4179843c26d54bd999a", 3, "file.txt" },
+		{ 0160000, "0000000000000000000000000000000000000001", 1, "submodule1" },
+		{ 0160000, "0000000000000000000000000000000000000002", 3, "submodule1" },
+		{ 0160000, "0000000000000000000000000000000000000003", 0, "submodule2" },
+	};
+
+	cl_git_pass(merge_trees_from_branches(&index, repo,
+		"submodule_rename1", "submodule_rename2",
+		opts));
+	cl_assert(merge_test_index(index, merge_index_entries, 7));
+	git_index_free(index);
+}