submodule-config.c: use repo_get_oid for reading .gitmodules

submodule-config.c: use repo_get_oid for reading .gitmodules

Since 76e9bdc437 (submodule: support reading .gitmodules when it's not
in the working tree - 2018-10-25), every time you do

    git grep --recurse-submodules

you are likely to see one warning line per submodule (unless all those
submodules also have submodules). On a superproject with plenty of
submodules (I've seen one with 67) this is really annoying.

The warning was there because we could not resolve extended SHA-1
syntax on a submodule. We can now. Make use of the new API and get rid
of the warning.

It would be even better if config_with_options() supports multiple
repositories too. But one step at a time.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>