submodule-helper: fix indexing in clone retry error reporting path

submodule-helper: fix indexing in clone retry error reporting path

'git submodule--helper update-clone' has logic to retry failed clones
a second time. For this purpose, there is a list of submodules to clone,
and a second list that is filled with the submodules to retry. Within
these lists, the submodules are identified by an index as if both lists
were just appended.

This works nicely except when the second clone attempt fails as well. To
report an error, the identifying index must be adjusted by an offset so
that it can be used as an index into the second list. However, the
calculation uses the logical total length of the lists so that the result
always points one past the end of the second list.

Pick the correct index.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Acked-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>