shell: add basic tests

We have no tests of even basic functionality of git-shell. Let's add a
couple of obvious ones. This will serve as a framework for adding tests
for new things we fix, as well as making sure we don't screw anything up
too badly while doing so.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

t/t9850-shell.sh

@@ -0,0 +1,31 @@
+#!/bin/sh
+
+test_description='git shell tests'
+. ./test-lib.sh
+
+test_expect_success 'shell allows upload-pack' '
+	printf 0000 >input &&
+	git upload-pack . <input >expect &&
+	git shell -c "git-upload-pack $SQ.$SQ" <input >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'shell forbids other commands' '
+	test_must_fail git shell -c "git config foo.bar baz"
+'
+
+test_expect_success 'shell forbids interactive use by default' '
+	test_must_fail git shell
+'
+
+test_expect_success 'shell allows interactive command' '
+	mkdir git-shell-commands &&
+	write_script git-shell-commands/ping <<-\EOF &&
+	echo pong
+	EOF
+	echo pong >expect &&
+	echo ping | git shell >actual &&
+	test_cmp expect actual
+'
+
+test_done