hash: use git_hash_init() consistently

hash: use git_hash_init() consistently

We'd like to add more logic to git_hash_init(), but many callers skip it
and call algop->init_fn() directly. Let's make sure we're consistently
using the wrapper by adding a coccinelle rule.

Besides the coccinelle file itself, this is a purely mechanical
conversion based on the patch it generates. There should be no bare
init_fn() calls left (except for the one in the wrapper).

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