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

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>