Merge branch 'gc/submodule-update-part2'

"git submodule update" without pathspec should silently skip an
uninitialized submodule, but it started to become noisy by mistake.

* gc/submodule-update-part2:
  submodule--helper: fix initialization of warn_if_uninitialized

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