Make sure different git references get different folders

Hi there,

I use the package [shama/napa](https://github.com/shama/napa), installing a lot from tagged private git repos. Alas, it turns out that different tags land in the same cached folder. Napa always installs the cached version when available – and in effect it's impossible to update any libraries!

I don't know if this change goes inline with the package's intent. If it does, please merge.

Cheers

P.S. Just wondering if the double semicolons are some sort of irony :) I've remembered you by a blog post about semicolon-freedom :)