fetchgit: add preFetch hook (#404488)

: Whether to fetch LFS objects.

*`preFetch`* (String)

: Shell code to be executed before the repository has been fetched, to allow

  changing the environment the fetcher runs in.

*`postFetch`* (String)

: Shell code executed after the file has been fetched successfully.

: Shell code executed after the repository has been fetched successfully.

  This can do things like check or transform the file.

*`leaveDotGit`* (Boolean)

echo "exporting $url (rev $rev) into $out"

runHook preFetch

$SHELL $fetcher --builder --url "$url" --out "$out" --rev "$rev" --name "$name" \

  ${leaveDotGit:+--leave-dotGit} \

  ${fetchLFS:+--fetch-lfs} \

      nonConeMode ? false,

      name ? null,

      nativeBuildInputs ? [ ],

      # Shell code executed before the file has been fetched.  This, in

      # particular, can do things like set NIX_PREFETCH_GIT_CHECKOUT_HOOK to

      # run operations between the checkout completing and deleting the .git

      # directory.

      preFetch ? "",

      # Shell code executed after the file has been fetched

      # successfully. This can do things like check or transform the file.

      postFetch ? "",

      server admins start using the new version?

    */

    assert deepClone -> leaveDotGit;

    assert nonConeMode -> (sparseCheckout != [ ]);

    let

          deepClone

          branchName

          nonConeMode

          preFetch

          postFetch

          ;

        rev = revWithTag;