submodule--helper: replace connect-gitdir-workingtree by ensure-core-worktree

submodule--helper: replace connect-gitdir-workingtree by ensure-core-worktree

e98317508c0 (submodule: ensure core.worktree is set after update,
2018-06-18) was overly aggressive in calling connect_work_tree_and_git_dir
as that ensures both the 'core.worktree' configuration is set as well as
setting up correct gitlink file pointing at the git directory.

We do not need to check for the gitlink in this part of the cmd_update
in git-submodule.sh, as the initial call to update-clone will have ensured
that. So we can reduce the work to only (check and potentially) set the
'core.worktree' setting.

While at it move the check from shell to C as that proves to be useful in
a follow up patch, as we do not need the 'name' in shell now.

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