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