raco pkg clone: avoid fetching by commit id
raco pkg clone: avoid fetching by commit id

A commit id apparently doesn't work in place of a <refexp> in some
versions of Git.

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