tests: test terminal output to both stdout and stderr

Some outputs (like the pager) care whether stdout is a
terminal. Others (like progress meters) care about stderr.

This patch sets up both. Technically speaking, we could go
further and set up just one (because either the other goes
to a terminal, or because our tests are only interested in
one). This patch does both to keep the interface to
lib-terminal simple.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-terminal.sh

@@ -1,19 +1,19 @@
 #!/bin/sh
 
 test_expect_success 'set up terminal for tests' '
-	if test -t 1
+	if test -t 1 && test -t 2
 	then
-		>stdout_is_tty
+		>have_tty
 	elif
 		test_have_prereq PERL &&
 		"$PERL_PATH" "$TEST_DIRECTORY"/test-terminal.perl \
-			sh -c "test -t 1"
+			sh -c "test -t 1 && test -t 2"
 	then
 		>test_terminal_works
 	fi
 '
 
-if test -e stdout_is_tty
+if test -e have_tty
 then
 	test_terminal() { "$@"; }
 	test_set_prereq TTY