submodule--helper: libify more "die" paths for module_update()

submodule--helper: libify more "die" paths for module_update()

As noted in a preceding commit the get_default_remote_submodule() and
remote_submodule_branch() functions would invoke die(), and thus leave
update_submodule() only partially lib-ified. Let's address the former
of those cases.

Change the functions to return an int exit code (non-zero on failure),
while leaving the get_default_remote() function for the callers that
still want the die() semantics.

This change addresses 1/2 of the "die" issue in these two lines in
update_submodule():

	char *remote_name = get_default_remote_submodule(update_data->sm_path);
	const char *branch = remote_submodule_branch(update_data->sm_path);

We can safely remove the "!default_remote" case from sync_submodule(),
because our get_default_remote_submodule() function now returns a
die_message() on failure, so we can have it and other callers check if
the exit code should be non-zero instead.

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