submodule: use "fetch" logic instead of custom remote discovery

Replace a use of the get_default_remote() function with an invocation
of "git fetch"

The "fetch" command already has logic to discover the remote for the
current branch. However, before it learned to accept a custom
refspec *and* use its idea of the default remote, it wasn't possible
to get rid of some equivalent of the "get_default_remote" invocation
here.

As it turns out the recently added "--stdin" option to fetch[1] gives
us a way to do that. Let's use it instead.

While I'm at it simplify the "fetch_in_submodule" function. It wasn't
necessary to pass "$@" to "fetch" since we'd only ever provide one
SHA-1 as an argument in the previous "*" codepath (in addition to
"--depth=N"). Rewrite the function to more narrowly reflect its
use-case.

1. https://lore.kernel.org/git/87eekwf87n.fsf@evledraar.gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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