sparse-checkout: add '--stdin' option to set subcommand

sparse-checkout: add '--stdin' option to set subcommand

The 'git sparse-checkout set' subcommand takes a list of patterns
and places them in the sparse-checkout file. Then, it updates the
working directory to match those patterns. For a large list of
patterns, the command-line call can get very cumbersome.

Add a '--stdin' option to instead read patterns over standard in.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>