Backpropagate Cargo.lock updates to all lock files (#9180)

* Experiment to backpropagate Cargo.lock updates to all lock files

* Move most of dependabot-specific code to its own file

* Various cleanups

* Fine tune..

* Clean up shells and stop obscure API...

scripts/cargo-for-all-lock-files.sh

@@ -0,0 +1,38 @@
+#!/usr/bin/env bash
+
+set -e
+
+shifted_args=()
+while [[ -n $1 ]]; do
+  if [[ $1 = -- ]]; then
+    escape_marker=found
+    shift
+    break
+  else
+    shifted_args+=("$1")
+    shift
+  fi
+done
+
+# When "--" appear at the first and shifted_args is empty, consume it here
+# to unambiguously pass and use any other "--" for cargo
+if [[ -n $escape_marker && ${#shifted_args[@]} -gt 0 ]]; then
+  files="${shifted_args[*]}"
+  for file in $files; do
+    if [[ $file = "${file%Cargo.lock}" ]]; then
+      echo "$0: unrecognizable as Cargo.lock path (prepend \"--\"?): $file" >&2
+      exit 1
+    fi
+  done
+  shifted_args=()
+else
+  files="$(git ls-files :**/Cargo.lock)"
+fi
+
+for lock_file in $files; do
+  (
+    set -x
+    cd "$(dirname "$lock_file")"
+    cargo "${shifted_args[@]}" "$@"
+  )
+done