sparse-checkout: use improved unpack_trees porcelain messages

setup_unpack_trees_porcelain() provides much improved error/warning messages; instead of a message that assumes that there is only one path with a given problem despite being used by code that intentionally is grouping and showing errors together, it uses a message designed to be used with groups of paths. For example, this transforms error: Entry ' folder1/a folder2/a ' not uptodate. Cannot update sparse checkout. into error: Cannot update sparse checkout: the following entries are not up to date: folder1/a folder2/a In the past the suboptimal messages were never actually triggered because we would error out if the working directory wasn't clean before we even called unpack_trees(). The previous commit changed that, though, so let's use the better error messages. Reviewed-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>