sparse-checkout: pass use_stdin as a parameter instead of as a global

sparse-checkout: pass use_stdin as a parameter instead of as a global

add_patterns_from_input() has relied on a global variable,
set_opts.use_stdin, which has been used by both the `set` and `add`
subcommands of sparse-checkout.  Once we introduce an
add_opts.use_stdin, the hardcoding of set_opts.use_stdin will be
incorrect.  Pass the value as function parameter instead to allow us to
make subsequent changes.

Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Victoria Dye <vdye@github.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>