raco pkg: improve Git repo-download caching
raco pkg: improve Git repo-download caching

Drop the path and branch from a URL that references a Git
repository, so that a single download is used for multiple
packages in the same repo.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket