parallel-checkout: add tests related to .gitattributes

parallel-checkout: add tests related to .gitattributes

Add tests to confirm that the `struct conv_attrs` data is correctly
passed from the main process to the workers, and that they can properly
convert the blobs before writing them to the working tree.

Also check that parallel-ineligible entries, such as regular files that
require external filters, are correctly smudge and written when
parallel-checkout is enabled.

Co-authored-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>