t: use test_write_lines() instead of series of 'echo' commands

These tests employ a noisy subshell (with missing &&-chain) to feed
input into Git commands or files:

    (echo a; echo b; echo c) | git some-command ...

Simplify by taking advantage of test_write_lines():

    test_write_lines a b c | git some-command ...

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0090-cache-tree.sh

@@ -156,7 +156,7 @@ test_expect_success PERL 'commit --interactive gives cache-tree on partial commi
 		return 44;
 	}
 	EOT
-	(echo p; echo 1; echo; echo s; echo n; echo y; echo q) |
+	test_write_lines p 1 "" s n y q |
 	git commit --interactive -m foo &&
 	test_cache_tree
 '