The hashmap API provides hashmap_iter_first() helper for initialion
and getting the first entry of a hashmap. Let's use it instead of
doing initialization manually and then get the first entry.
There are no functional changes, just cleanup.
Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com> Reviewed-by: Stefan Beller <sbeller@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>