nix-prefetch-git: use fetchgit's naming heuristic

This commit fixes #6651.

Before this change the `nix-prefetch-git` script would use a different store
name than nix's `fetchgit` function. Because of that it was not possible to
use `nix-prefetch-git` as a way to pre-populate the store (for example when
the user it using private git dependencies that needs access to the ssh agent)