>> I don't know what I should do here.  Both Stefan and Lars think (IIUC) 
>> that it's okay to use the filename as is.
>
> No: the hash table should be indexed by absolute file names. Anything 
> else will lead to bugs. There are various ways to get those names to be 
> absolute, with tradeoffs.
>

Okay, so there were three opinions, not two ;-)

Here's an again improved version, which I hope is a reasonable compromise.