test-hashmap: use "unsigned int" for hash storage

test-hashmap: use "unsigned int" for hash storage

The hashmap API always use an unsigned value for storing
and comparing hashes. Whereas this test code uses "int".
This works out in practice since one can typically
round-trip between "int" and "unsigned int". But since this
is essentially reference code for the hashmap API, we should
model using the correct types.

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