convert: add get_stream_filter_ca() variant

convert: add get_stream_filter_ca() variant

Like the previous patch, we will also need to call get_stream_filter()
with a precomputed `struct conv_attrs`, when we add support for parallel
checkout workers. So add the _ca() variant which takes the conversion
attributes struct as a parameter.

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