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

1 files changed, 2 insertions, 2 deletions

diff --git a/src/libfetchers/github.cc b/src/libfetchers/github.cc
index 4c1a140ff..8bb7c2c1d 100644
--- a/src/libfetchers/github.cc
+++ b/src/libfetchers/github.cc
@@ -229,13 +229,13 @@ struct GitLabInputScheme : GitArchiveInputScheme
     Hash getRevFromRef(nix::ref<Store> store, const Input & input) const override
     {
         auto host_url = maybeGetStrAttr(input.attrs, "url").value_or("gitlab.com");
-        auto url = fmt("https://%s/api/v4/projects/%s%%2F%s/repository/branches/%s",
+        auto url = fmt("https://%s/api/v4/projects/%s%%2F%s/repository/commits?ref_name=%s",
             host_url, getStrAttr(input.attrs, "owner"), getStrAttr(input.attrs, "repo"), *input.getRef());
         auto json = nlohmann::json::parse(
             readFile(
                 store->toRealPath(
                     downloadFile(store, url, "source", false).storePath)));
-        auto rev = Hash(std::string(json["commit"]["id"]), htSHA1);
+        auto rev = Hash(std::string(json[0]["id"]), htSHA1);
         debug("HEAD revision for '%s' is %s", url, rev.gitRev());
         return rev;
     }