t: handle EOF in test_copy_bytes()

t: handle EOF in test_copy_bytes()

The test_copy_bytes() function claims to read up to N bytes,
or until it gets EOF. But we never handle EOF in our loop,
and a short input will cause perl to go into an infinite
loop of read() getting zero bytes.

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