submodule: extract get_fetch_task()

submodule: extract get_fetch_task()

get_next_submodule() configures the parallel submodule fetch by
performing two functions:

* iterate the index to find submodules
* configure the child processes to fetch the submodules found in the
  previous step

Extract the index iterating code into an iterator function,
get_fetch_task(), so that get_next_submodule() is agnostic of how
to find submodules. This prepares for a subsequent commit will teach the
fetch machinery to also iterate through the list of changed
submodules (in addition to the index).

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>