Add BUILD_OFFLINE capability to skip post-checkout submodule init

Patch by Josh McKenzie; reviewed by David Capwell for CASSANDRA-21364

.build/git/git-hooks/post-checkout/100-update-submodules.sh

@@ -31,11 +31,18 @@ _main() {
   root_dir="$(git rev-parse --show-toplevel)"
   cd "$root_dir"
 
-  if [[ ! -e .gitmodules ]]; then
+  if ! git ls-files --error-unmatch .gitmodules &>/dev/null; then
     # nothing to see here, look away!
     return 0
   fi
-  git submodule update --init --recursive
+
+  local extra_args=()
+  if [[ -n "${BUILD_OFFLINE:-}" ]]; then
+    echo "BUILD_OFFLINE is defined; using --no-fetch for submodule updates (local SHAs only)."
+    echo "If your submodules are not already available locally, expect this to error out."
+    extra_args+=(--no-fetch)
+  fi
+  git submodule update --init --recursive "${extra_args[@]}"
 }
 
 _main "$@"

.build/git/git-hooks/pre-commit/100-verify-submodules-pushed.sh

@@ -60,7 +60,7 @@ _main() {
   root_dir="$(git rev-parse --show-toplevel)"
   cd "$root_dir"
 
-  [[ ! -e .gitmodules ]] && return 0
+  git ls-files --error-unmatch .gitmodules &>/dev/null || return 0
   local enabled=$(git config --bool cassandra.pre-commit.verify-submodules.enabled || echo true)
   [ "$enabled" == "false" ] && return 0
   local submodules=( $(git config --file .gitmodules --get-regexp path | awk '{ print $2 }') )