fetchgit: add preFetch hook

This allows running code to change the environment before the
nix-prefetch-git script is run.  In particular, it allows setting things
like NIX_PREFETCH_GIT_CHECKOUT_HOOK, which allows additional code to be
run before deleting the .git directory.

This also means there's potentially value in performing a deep clone
then removing the .git directory, so remove the assertion that prevents
doing so.

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