refspec: replace `refspec_item_init()` with fetch/push variants

refspec: replace `refspec_item_init()` with fetch/push variants

For similar reasons as in the previous refactoring of `refspec_init()`
into `refspec_init_fetch()` and `refspec_init_push()`, apply the same
refactoring to `refspec_item_init()`.

Signed-off-by: Taylor Blau <redacted>
Acked-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>