submodule--helper: don't recreate recursive prefix

submodule--helper: don't recreate recursive prefix

update_submodule() uses duplicated code to compute
update_data->displaypath and next.recursive_prefix. The latter is just
the former with "/" appended to it, and since update_data->displaypath
not changed outside of this statement, we can just reuse the already
computed result.

We can go one step further and remove the reference to
next.recursive_prefix altogether. Since it is only used in
update_data_to_args() (to compute the "--recursive-prefix" flag for the
recursive update child process) we can just use the already computed
.displaypath value of there.

Delete the duplicated code, and remove the unnecessary reference to
next.recursive_prefix. As a bonus, this fixes a memory leak where
prefixed_path was never freed (this leak was first reported in [1]).

[1] https://lore.kernel.org/git/877a45867ae368bf9e053caedcb6cf421e02344d.1655336146.git.gitgitgadget@gmail.com

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>