index-pack, unpack-objects: restore missing ->init_fn

Commit 0578f1e ("global: adapt callers to use generic hash context helpers")
accidentally removed `->init_fn`, which is required for OpenSSL 3+ SHA1.

This fixes the following error on fetch:
  fatal: fetch-pack: invalid index-pack output

Signed-off-by: Jensen Huang <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Jensen Huang

builtin/index-pack.c

@@ -1286,6 +1286,7 @@ static void parse_pack_objects(unsigned char *hash)
 
 	/* Check pack integrity */
 	flush();
+	the_hash_algo->init_fn(&tmp_ctx);
 	git_hash_clone(&tmp_ctx, &input_ctx);
 	git_hash_final(hash, &tmp_ctx);
 	if (!hasheq(fill(the_hash_algo->rawsz), hash, the_repository->hash_algo))

builtin/unpack-objects.c

@@ -668,6 +668,7 @@ int cmd_unpack_objects(int argc,
 	the_hash_algo->init_fn(&ctx);
 	unpack_all();
 	git_hash_update(&ctx, buffer, offset);
+	the_hash_algo->init_fn(&tmp_ctx);
 	git_hash_clone(&tmp_ctx, &ctx);
 	git_hash_final_oid(&oid, &tmp_ctx);
 	if (strict) {