Merge branch 'gc/submodule-update-part2' into maint

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

This fixes a regression in 2.36 and is slate to go to 2.36.1
source: <pull.1258.v2.git.git.1650890741430.gitgitgadget@gmail.com>

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

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