data: Dereference refs when reading and writing

Now we'll dereference symbolic refs not only when reading them but also when
writing them.

We'll implement a helper function called `_get_ref_internal` which will return
the path and the value of the last ref pointed by a symbolic ref. In simple words:

- When given a non-symbolic ref, `_get_ref_internal` will return the ref name
and value.
- When given a symbolic ref, `_get_ref_internal` will dereference the ref
recursively, and then return the name of the last (non-symbolic) ref that points
to an OID, plus its value.

Now `update_ref` will use `_get_ref_internal` to know which ref it needs to update.

Additionally, we'll use `_get_ref_internal` in `get_ref`.