- tag: Resolve name to oid in argparse

It's nice that we can create tags, but now let's actually make them usable from
the CLI.

In *base.py*, we'll create `get_oid` to resolve a "name" to an OID. A name can
either be a ref (in which case `get_oid` will return the OID that the ref points
to) or an OID (in which case `get_oid` will just return that same OID).

Next, we'll modify the argument parser in *cli.py* to call `get_oid` on all
arguments which are expected to be an OID. This way we can pass a ref there
instead of an OID.

At this point we can do something like:
```
$ ugit tag mytag d8d43b0e3a21df0c845e185d08be8e4028787069
$ ugit log refs/tags/mytag
# Will print log of commits starting at d8d43b0e...
$ ugit checkout refs/tags/mytag
# Will checkout commit d8d43b0e...
etc...
```