Merge branch 'sb/submodule-clone-retry'

An earlier tweak to make "submodule update" retry a failing clone
of submodules was buggy and caused segfault, which has been fixed.

* sb/submodule-clone-retry:
submodule-helper: fix indexing in clone retry error reporting path
git-submodule: forward exit code of git-submodule--helper more faithfully

diff --cc git-submodule.sh
Simple merge