sparse-checkout: optimize string_list construction and add tests to verify deduplication.

sparse-checkout: optimize string_list construction and add tests to verify deduplication.

Improve O(n^2) complexity to O(n log n) while building a sorted
'string_list' by constructing it unsorted then sorting it
followed by removing duplicates.

sparse-checkout deduplicates repeated cone-mode patterns,
but this behaviour was previously untested, add tests that
verify that sparse-checkout file contain each cone
pattern only once and sparse-checkout list reports each pattern
only once.

Signed-off-by: Amisha Chhajed <amishhhaaaa@gmail.com>
Acked-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>