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>

Documentation/git-sparse-checkout.txt

@@ -47,6 +47,9 @@ To avoid interfering with other worktrees, it first enables the
 	a list of arguments following the 'set' subcommand. Update the
 	working directory to match the new patterns. Enable the
 	core.sparseCheckout config setting if it is not already enabled.
++
+When the `--stdin` option is provided, the patterns are read from
+standard in as a newline-delimited list instead of from the arguments.
 
 SPARSE CHECKOUT
 ---------------