submodule--helper: add "const" to passed "module_clone_data"

submodule--helper: add "const" to passed "module_clone_data"

Add "const" to the "struct module_clone_data" that we pass to
clone_submodule(), which makes the ownership clear, and stops us from
clobbering the "clone_data->path".

We still need to add to the "reference" member, which is a "struct
string_list". Let's do this by having clone_submodule() create its
own, and copy the contents over, allowing us to pass it as a
separate parameter.

This new "struct string_list" still leaks memory, just as the "struct
module_clone_data" did before. let's not fix that for now, to fix that
we'll need to add some "goto cleanup" to the relevant code. That will
eventually be done in follow-up commits, this change makes it easier
to fix the memory leak.

The scope of the new "reference" variable in add_submodule() could be
narrowed to the "else" block, but as we'll eventually free it with a
"goto cleanup" let's declare it at the start of the function.

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>