submodule--helper: use submodule_from_path in set-{url,branch}

submodule--helper: use submodule_from_path in set-{url,branch}

The commands need a path to a submodule but treated it as the name when
modifying the .gitmodules file, leading to confusion when a submodule's
name does not match its path.

Because calling submodule_from_path initializes the submodule cache, we
need to manually trigger a reread before syncing, as the cache is
missing the config change we just made.

Signed-off-by: Jan Alexander Steffens (heftig) <heftig@archlinux.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>