When deciding whether to add the `**/` prefix or not, we should choose
not to add it if the pattern is simply a bare `**`. Previously, we were
only not adding it if it was `**/`, which is correct, but we also need
to do it for `**` since `**` can already match anywhere.
There's likely a more principled solution to this, but this works for
now.
Fixes#1173
23be3cf850