Fix gitlab-fetcher to obtain tags and branches

Until now, the `gitlab`-fetcher determined the source's rev by checking
the latest commit of the given `ref` using the
`/repository/branches`-API.

This breaks however when trying to fetch a gitlab-repo by its tag:

```
$ nix repl
nix-repl> builtins.fetchTree gitlab:Ma27/nvim.nix/0.2.0
error: --- Error ------------------------------------------------------------------------------------- nix
unable to download 'https://gitlab.com/api/v4/projects/Ma27%2Fnvim.nix/repository/branches/0.2.0': HTTP error 404 ('')
```

When using the `/commits?ref_name`-endpoint[1] you can pass any kind of
valid ref to the `gitlab`-fetcher.

Please note that this fetches the only first 20 commits on a ref,
unfortunately there's currently no endpoint which only retrieves the
latest commit of any kind of `ref`.

[1] https://docs.gitlab.com/ee/api/commits.html#list-repository-commits

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