nix-prefetch-git: make sure the script is interpreted by bash

Fixes https://github.com/NixOS/nixpkgs/issues/11284.

pkgs/build-support/fetchgit/nix-prefetch-git

@@ -1,4 +1,6 @@
-#! /bin/sh -e
+#! /usr/bin/env bash
+
+set -e -o pipefail
 
 url=
 rev=