server: actually use git repository updates

before this, the server would fetch updates but then not update the
corresponding reference, leading to the initial state being checked out
and linted again.

server/Worker.hs

@@ -93,7 +93,7 @@ runJob offline config Job {..} done = do
         True | offline -> logInfoN $ "offline mode: not updating " <> show gitdir
              | otherwise ->
           (liftIO $ callgit gitdir
-            [ "fetch", "origin", toString ref, "--depth", "1" ])
+            [ "fetch", "origin", toString (ref <> ":" <> ref) ])
       rev <- map T.strip -- git returns a newline here
         $ readgit' gitdir ["rev-parse", toString ref]