Use "--prune" when fetching

This is needed to make sure that we don't try to delete a remote branch
that no longer exists. This fixes arc90#18.

git-sweep

@@ -66,7 +66,7 @@ def git(arguments, path):
 
 def fetch(remote, path):
     """Download objects and refs from remote repository."""
-    git(['fetch', remote], path=path)
+    git(['fetch', '--prune', remote], path=path)
 
 
 def remote_heads(remote, path):