refs/files: stop using the_repository in parse_loose_ref_contents()

We implicitly rely on `the_repository` in `parse_loose_ref_contents()`
by calling `parse_oid_hex()`. Convert the function to instead use
`parse_oid_hex_algop()` and have callers pass in the hash algorithm to
use.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -1752,8 +1752,8 @@ static int refs_read_special_head(struct ref_store *ref_store,
 		goto done;
 	}
 
-	result = parse_loose_ref_contents(content.buf, oid, referent, type,
-					  failure_errno);
+	result = parse_loose_ref_contents(ref_store->repo->hash_algo, content.buf,
+					  oid, referent, type, failure_errno);
 
 done:
 	strbuf_release(&full_path);