test-hashmap: use strbuf_getline rather than fgets

test-hashmap: use strbuf_getline rather than fgets

Using fgets() with a fixed-size buffer can lead to lines
being accidentally split across two calls if they are larger
than the buffer size.

As this is just a test helper, this is unlikely to be a
problem in practice. But since people may look at test
helpers as reference code, it's a good idea for them to
model the preferred behavior.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>