Use new HASHMAP_INIT macro to simplify hashmap initialization

Use new HASHMAP_INIT macro to simplify hashmap initialization

Now that hashamp has lazy initialization and a HASHMAP_INIT macro,
hashmaps allocated on the stack can be initialized without a call to
hashmap_init() and in some cases makes the code a bit shorter.  Convert
some callsites over to take advantage of this.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>