Merge branch 'sb/submodule-clone-retry'

"git submodule update" that drives many "git clone" could
eventually hit flaky servers/network conditions on one of the
submodules; the command learned to retry the attempt.

* sb/submodule-clone-retry:
  submodule update: continue when a clone fails
  submodule--helper: initial clone learns retry logic

diff --cc builtin/submodule--helper.c
Simple merge

diff --cc git-submodule.sh
Simple merge