fetchGit: fix creation of uninitialized cache dir, let git create it
fetchGit: fix creation of uninitialized cache dir, let git create it

fetchGit test (as modified in previous commit) now passes.

pikajude/nix,NixOS/nix,NixOS/nix,dezgeg/nix,dtzWill/nix,dezgeg/nix,layus/nix,zimbatm/nix,ehmry/nix,shlevy/nix,zimbatm/nix,zimbatm/nix,ehmry/nix,rycee/nix,dtzWill/nix,zimbatm/nix,vcunat/nix,peti/nix,peti/nix,dezgeg/nix,NixOS/nix,layus/nix,dezgeg/nix,dtzWill/nix,vcunat/nix,shlevy/nix,rycee/nix,rycee/nix,NixOS/nix,pikajude/nix,vcunat/nix,peti/nix,shlevy/nix,layus/nix,layus/nix,layus/nix,dezgeg/nix,dtzWill/nix,vcunat/nix,ehmry/nix,pikajude/nix,rycee/nix,ehmry/nix,peti/nix,dtzWill/nix,rycee/nix,peti/nix,NixOS/nix,ehmry/nix,shlevy/nix,shlevy/nix,ehmry/nix