ref: store the target in the record

There's no need to create a new object every time the user asks for
the target, so store it the same way we do for the type.

src/geef_records.hrl

@@ -1,4 +1,4 @@
--record(ref, {handle, type :: atom()}).
+-record(ref, {handle, type :: atom(), target :: binary() | oid()}).
 -record(repo, {handle}).
 -record(odb, {handle}).
 -record(revwalk, {handle}).