submodule: migrate get_next_submodule to use repository structs

submodule: migrate get_next_submodule to use repository structs

We used to recurse into submodules, even if they were broken having
only an objects directory. The child process executed in the submodule
would fail though if the submodule was broken. This is tested via
"fetching submodule into a broken repository" in t5526.

This patch tightens the check upfront, such that we do not need
to spawn a child process to find out if the submodule is broken.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>