- tag: Create the tag ref

After we've implemented refs in the previous change, it's time to create a ref
when the user creates a tag.

`create_tag` now calls update_ref with the tag name to actually create the tag.

For namespacing purposes, we'll put all tags under *refs/tags/*. That is, if the
user creates *my-cool-commit* tag, we'll create *refs/tags/my-cool-commit* ref
to point to the desired OID.

Then we'll update *data.py* to handle this "namespaced" ref. Since we can't have
a / in the file name, we'll create directories for it. Now if a ref
*refs/tags/sometag* is created, it will be placed under *.ugit/refs/tags* in a
file named *sometag*.

To verify that this code works, you can run:
```
$ ugit tag test
```

And make sure that the tag points to HEAD:
```
$ cat .ugit/refs/tags/test
$ cat .ugit/HEAD
```

The last two commands should give the same output.