hashmap: use *_entry APIs for iteration

Inspired by list_for_each_entry in the Linux kernel.
Once again, these are somewhat compromised usability-wise
by compilers lacking __typeof__ support.

Signed-off-by: Eric Wong <e@80x24.org>
Reviewed-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-hashmap.c

@@ -222,10 +222,11 @@ int cmd__hashmap(int argc, const char **argv)
 			free(entry);
 
 		} else if (!strcmp("iterate", cmd)) {
-
 			struct hashmap_iter iter;
-			hashmap_iter_init(&map, &iter);
-			while ((entry = hashmap_iter_next(&iter)))
+
+			hashmap_for_each_entry(&map, &iter, entry,
+						struct test_entry,
+						ent /* member name */)
 				printf("%s %s\n", entry->key, get_value(entry));
 
 		} else if (!strcmp("size", cmd)) {