round-trip trees through index_read_index

Read a tree into an index using `git_index_read_index` (by reading
a tree into a new index, then reading that index into the current
index), then write the index back out, ensuring that our new index
is treesame to the tree that we read.

tests/index/read_index.c

@@ -71,3 +71,35 @@ void test_index_read_index__maintains_stat_cache(void)
 		}
 	}
 }
+
+static bool roundtrip_with_read_index(const char *tree_idstr)
+{
+	git_oid tree_id, new_tree_id;
+	git_tree *tree;
+	git_index *tree_index;
+
+	cl_git_pass(git_oid_fromstr(&tree_id, tree_idstr));
+	cl_git_pass(git_tree_lookup(&tree, _repo, &tree_id));
+	cl_git_pass(git_index_new(&tree_index));
+	cl_git_pass(git_index_read_tree(tree_index, tree));
+	cl_git_pass(git_index_read_index(_index, tree_index));
+	cl_git_pass(git_index_write_tree(&new_tree_id, _index));
+
+	git_tree_free(tree);
+	git_index_free(tree_index);
+
+	return git_oid_equal(&tree_id, &new_tree_id);
+}
+
+void test_index_read_index__produces_treesame_indexes(void)
+{
+	roundtrip_with_read_index("53fc32d17276939fc79ed05badaef2db09990016");
+	roundtrip_with_read_index("944c0f6e4dfa41595e6eb3ceecdb14f50fe18162");
+	roundtrip_with_read_index("1810dff58d8a660512d4832e740f692884338ccd");
+	roundtrip_with_read_index("d52a8fe84ceedf260afe4f0287bbfca04a117e83");
+	roundtrip_with_read_index("c36d8ea75da8cb510fcb0c408c1d7e53f9a99dbe");
+	roundtrip_with_read_index("7b2417a23b63e1fdde88c80e14b33247c6e5785a");
+	roundtrip_with_read_index("f82a8eb4cb20e88d1030fd10d89286215a715396");
+	roundtrip_with_read_index("fd093bff70906175335656e6ce6ae05783708765");
+	roundtrip_with_read_index("ae90f12eea699729ed24555e40b9fd669da12a12");
+}