Add cherry-pick.sh convenience script.

src/tools/cherry-pick.sh

@@ -0,0 +1,34 @@
+#!/bin/bash
+set -euo pipefail
+IFS=$'\n\t'
+
+print_error() {
+  echo "Error: \`$1\` is not a valid commit. To debug, run:"
+  echo
+  echo "    git rev-parse --verify $1"
+  echo
+}
+
+full_sha() {
+  git rev-parse \
+    --verify \
+    --quiet \
+    "$1^{object}" || print_error $1
+}
+
+commit_message_with_backport_note() {
+  message=$(git log --format=%B -n 1 $1)
+  echo $message | awk "NR==1{print; print \"\n(backport-of: $1)\"} NR!=1"
+}
+
+cherry_pick_commit() {
+  sha=$(full_sha $1)
+  git cherry-pick $sha > /dev/null
+  git commit \
+    --amend \
+    --file <(commit_message_with_backport_note $sha)
+}
+
+for arg ; do
+  cherry_pick_commit $arg
+done