Add some tests for Index

Including making the default :undefined, so the NIF doesn't get
upset. Implementing the Access protocol would require a struct, so we
may or may not want that.

test/index_test.exs

@@ -0,0 +1,44 @@
+defmodule IndexTest do
+  use ExUnit.Case
+  use Geef
+  alias Geef.Index
+  alias Geef.Index.Entry
+  import RepoHelpers
+
+
+  setup do
+    {repo, path} = tmp_bare()
+    Process.link(repo)
+    on_exit(fn -> File.rm_rf!(path) end)
+    {:ok, [repo: repo, path: path]}
+  end
+
+  test "add an entry to the index", meta do
+    repo = meta[:repo]
+
+    content = "This is some text that will go in a file"
+    {:ok, odb} = Repository.odb(repo)
+    {:ok, id} = Odb.write(odb, content, :blob)
+    assert id == Oid.parse("c300118399f01fe52b316061b5d32beb27e0adfd")
+
+    {:ok, idx} = Index.new
+    {now_mega, now_secs, _} = :os.timestamp()
+    time = now_mega * 1000000 + now_secs
+
+    entry = %Geef.Index.Entry{mode: 0o100644, id: id, path: "README", size: byte_size(content), mtime: time}
+    :ok = Index.add(idx, entry)
+
+    {:ok, tree_id} = Index.write_tree(idx, repo)
+    expected = Oid.parse("5a20bbbf65ea75ad4d9f995d179156824ccca3a1")
+    assert tree_id == expected
+
+    {:ok, entry1} = Index.get(idx, "README", 0)
+#    entry2 = idx["README"]
+
+#    assert entry1 == entry2
+    assert entry1.size == byte_size(content)
+
+    Repository.stop(repo)
+  end
+
+end