submodule: move logic into fetch_task_create()

submodule: move logic into fetch_task_create()

get_fetch_task() gets a fetch task by iterating the index; a future
commit will introduce a similar function, get_fetch_task_from_changed(),
that gets a fetch task from the list of changed submodules. Both
functions are similar in that they need to:

* create a fetch task
* initialize the submodule repo for the fetch task
* determine the default recursion mode

Move all of this logic into fetch_task_create() so that it is no longer
split between fetch_task_create() and get_fetch_task(). This will make
it easier to share code with get_fetch_task_from_changed().

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