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>