builtin/submodule--helper: store update_clone information in a struct

builtin/submodule--helper: store update_clone information in a struct

The information that is printed for update_submodules in
'submodule--helper update-clone' and consumed by 'git submodule update'
is stored as a string per submodule. This made sense at the time of
48308681b07 (git submodule update: have a dedicated helper for cloning,
2016-02-29), but as we want to migrate the rest of the submodule update
into C, we're better off having access to the raw information in a helper
struct.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>