- tag: Generalize HEAD to refs

As part of implementing `tag`, we'll generalize the way we handle HEAD. If you
think about it, HEAD and tags are similar. They are both ways for ugit to attach
a name to an OID. In case of HEAD, the name is hardcoded by ugit; in case of
tags, the name will be provided by the user. It makes sense to handle them
similarly in *data.py*.

In *data.py*, let's extend the function `set_HEAD` and `get_HEAD` to
`update_ref` and `get_ref`. "Ref" is a short for reference, and that's the name
Git uses. The function will now accept the name of the ref and write/read it as
a file under *.ugit* directory. Logically, a ref is a named pointer to an object.

The important change is in *data.py*. The rest of the changes just rename some
functions:

```
- get_HEAD()    ->   get_ref('HEAD')
- set_HEAD(oid) ->   update_ref('HEAD', oid)
```

Note that we didn't change any behaviour of ugit here, this is purely
refactoring.