submodule--helper: fix a leak in "clone_submodule"

Fix a memory leak of the "clone_data_path" variable that we copy or
derive from the "struct module_clone_data" in clone_submodule(). This
code was refactored in preceding commits, but the leak has been with
us since f8eaa0ba98 (submodule--helper, module_clone: always operate
on absolute paths, 2016-03-31).

For the "else" case we don't need to xstrdup() the "clone_data->path",
and we don't need to free our own "clone_data_path". We can therefore
assign the "clone_data->path" to our own "clone_data_path" right away,
and only override it (and remember to free it!) if we need to
xstrfmt() a replacement.

In the case of the module_clone() caller it's from "argv", and doesn't
need to be free'd, and in the case of the add_submodule() caller we
get a pointer to "sm_path", which doesn't need to be directly free'd
either.

Fixing this leak makes several tests pass, so let's mark them as
passing with TEST_PASSES_SANITIZE_LEAK=true.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7506-status-submodule.sh

@@ -2,6 +2,7 @@
 
 test_description='git status for submodule'
 
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 test_create_repo_with_commit () {