When reading the config of a submodule, if reading from a blob, read
using an explicitly specified repository instead of by adding the
submodule's ODB as an alternate and then reading an object from
the_repository.
This makes the "grep --recurse-submodules with submodules without
.gitmodules in the working tree" test in t7814 work when
GIT_TEST_FATAL_REGISTER_SUBMODULE_ODB is true.
Signed-off-by: Jonathan Tan <jonathantanmy@google.com> Reviewed-by: Matheus Tavares <matheus.bernardino@usp.br> Signed-off-by: Junio C Hamano <gitster@pobox.com>