- base: Try different directories when searching for a ref

In the previous change, you might have noticed that we need to spell out the
full name of a tag (Like *refs/tags/mytag*). This isn't very convenient, we
would like to have shorter command names. For example, if we've created "mytag"
tag, we should be able to do `ugit log mytag` rather than having to specify
`ugit log refs/tags/mytag`.

We'll extend `get_oid` to search in different ref subdirectories when resolving
a name. We'll search in:
```
    Root (.ugit): This way we can specify refs/tags/mytag
    .ugit/refs: This way we can specify tags/mytag
    .ugit/refs/tags: This way we can specify mytag
    .ugit/refs/heads: This will be needed for a future change
```
If we find the requested name in any of the directories, return it. Otherwise
assume that the name is an OID.