submodule-config: use hashmap_iter_first()

submodule-config: use hashmap_iter_first()

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 <redacted>
Reviewed-by: Stefan Beller <redacted>
Signed-off-by: Junio C Hamano <redacted>