Work towards elixir 14

While here, make the object be an elixir struct instead of delegating to
the erlang code.

As part of this, geef_nif:commit_tree() has been fixed to return the id,
insted of the type, as we know it's going to be a tree, but not then id.

test/test_helper.exs

@@ -6,6 +6,7 @@ defmodule RepoHelpers do
     n = node()
     dir = :io_lib.format("geef-~p~p~p~p.git", [n, a, b, c])
     path = Path.join(System.tmp_dir!, dir)
-    {Geef.Repository.init(path, true), path}
+    {:ok, repo} = Geef.Repository.init(path, true)
+    {repo, path}
   end
 end