fetchgit: make sparseCheckout a list of strings

The `sparseCheckout` argument allows the user to specify directories or
patterns of files, which Git uses to filter files it should check-out.

Git expects a multi-line string on stdin ("newline-delimited list", see
`git-sparse-checkout(1)`), but within nixpkgs it is more consistent to
use a list of strings instead. The list elements are joined to a
multi-line string only before passing it to the builder script.

A deprecation warning is emitted if a (multi-line) string is passed to
`sparseCheckout`, but for the time being it is still accepted.

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