Add lookup/2 to the specific types

They proxy to geef_object:lookup/3 which looks up the object and
asserts the type.

src/geef_commit.erl

@@ -1,5 +1,5 @@
 -module(geef_commit).
--export([tree_id/1, tree/1, id/1]).
+-export([tree_id/1, tree/1, id/1, lookup/2]).
 
 -include("geef_records.hrl").
 
@@ -16,5 +16,9 @@ tree(#object{type=commit,handle=Handle}) ->
 	    Other
     end.
 
+-spec lookup(repo(), oid() | iolist()) -> object().
+lookup(Repo, Id) ->
+    geef_object:lookup(Repo, Id, commit).
+
 id(Obj = #object{type=commit}) ->
     geef_object:id(Obj).