submodule: fix leaking submodule ODB paths

submodule: fix leaking submodule ODB paths

In `add_submodule_odb_by_path()` we add a path into a global string
list. The list is initialized with `NODUP`, which means that we do not
pass ownership of strings to the list. But we use `xstrdup()` when we
insert a path, with the consequence that the string will never get
free'd.

Plug the leak by marking the list as `DUP`. There is only a single
callsite where we insert paths anyway, and as explained above that
callsite was mishandling the allocation.

This leak is exposed by t7814, but plugging it does not make the whole
test suite pass.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>