grep: read submodule entry with explicit repo

grep: read submodule entry with explicit repo

Replace an existing parse_object_or_die() call (which implicitly works
on the_repository) with a function call that allows a repository to be
passed in. There is no such direct equivalent to parse_object_or_die(),
but we only need the type of the object, so replace with
oid_object_info().

Signed-off-by: Jonathan Tan <redacted>
Reviewed-by: Emily Shaffer <redacted>
Reviewed-by: Matheus Tavares <redacted>
Signed-off-by: Junio C Hamano <redacted>