hook: support a --to-stdin=<path> option

Expose the "path_to_stdin" API added in the preceding commit in the
"git hook run" command.

For now we won't be using this command interface outside of the tests,
but exposing this functionality makes it easier to test the hook
API. The plan is to use this to extend the "sendemail-validate"
hook[1][2].

1. https://lore.kernel.org/git/ad152e25-4061-9955-d3e6-a2c8b1bd24e7@amd.com
2. https://lore.kernel.org/git/20230120012459.920932-1-michael.strawbridge@amd.com

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1800-hook.sh

@@ -177,4 +177,22 @@ test_expect_success 'git hook run a hook with a bad shebang' '
 	test_cmp expect actual
 '
 
+test_expect_success 'stdin to hooks' '
+	write_script .git/hooks/test-hook <<-\EOF &&
+	echo BEGIN stdin
+	cat
+	echo END stdin
+	EOF
+
+	cat >expect <<-EOF &&
+	BEGIN stdin
+	hello
+	END stdin
+	EOF
+
+	echo hello >input &&
+	git hook run --to-stdin=input test-hook 2>actual &&
+	test_cmp expect actual
+'
+
 test_done