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>