submodule--helper update-clone: allow multiple references

submodule--helper update-clone: allow multiple references

Allow the user to pass in multiple references to update_clone.
Currently this is only internal API, but once the shell script is
replaced by a C version, this is needed.

This fixes an API bug between the shell script and the helper.
Currently the helper accepts "--reference" "--reference=foo"
as a OPT_STRING whose value happens to be "--reference=foo", and
then uses

        if (suc->reference)
                argv_array_push(&child->args, suc->reference)

where suc->reference _is_ "--reference=foo" when invoking the
underlying "git clone", it cancels out.

With this change we omit one of the "--reference" arguments when
passing references from the shell script to the helper.

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