contrib: add backport script

To ease the backport, I wrote a quick script.

Usage: ./backport.sh.sh stable-2.2
6892670d31 my-work

We can also pass multiple commits.
Follow up on @ktdreyer write up here:
https://github.com/ceph/ceph-ansible/pull/1529

Signed-off-by: Sébastien Han <seb@redhat.com>

contrib/backport_to_stable_branch.sh

@@ -0,0 +1,87 @@
+#!/usr/bin/env bash
+set -e
+shopt -s extglob # enable extended pattern matching features
+
+
+#############
+# VARIABLES #
+#############
+
+stable_branch=$1
+commit=$2
+bkp_branch_name=$3
+bkp_branch_name_prefix=bkp
+bkp_branch=$bkp_branch_name-$bkp_branch_name_prefix
+
+
+#############
+# FUNCTIONS #
+#############
+
+verify_commit () {
+  for com in ${commit//,/ }; do
+    if [[ $(git cat-file -t "$com" 2&>/dev/null) != commit ]]; then
+      echo "$com does not exist in your tree"
+      echo "Run 'git fetch origin master && git pull origin master'"
+      exit 1
+    fi
+  done
+}
+
+git_status () {
+  if [[ $(git status --porcelain | wc -l) -gt 0 ]]; then
+    echo "It looks like you have not committed changes:"
+    echo ""
+    git status --short
+    echo ""
+    echo ""
+    echo "Press ENTER to continue or Ctrl+c to break."
+    read -r
+  fi
+}
+
+checkout () {
+  git checkout --no-track -b "$bkp_branch" origin/"$stable_branch"
+}
+
+cherry_pick () {
+  local x
+  for com in ${commit//,/ }; do
+    x="$x $com"
+  done
+  # Trim the first white space and use an array
+  # Reference: https://github.com/koalaman/shellcheck/wiki/SC2086#exceptions
+  x=(${x##*( )})
+  git cherry-pick -x -s "${x[@]}"
+}
+
+push () {
+  git push origin "$bkp_branch"
+}
+
+cleanup () {
+  echo "Moving back to previous branch"
+  git checkout -
+  git branch -D "$bkp_branch"
+}
+
+test_args () {
+  if [ $# -ne 3 ]; then
+    echo "Please run the script like this: ./contrib/backport_to_stable_branch.sh STABLE_BRANCH_NAME COMMIT_SHA1 BACKPORT_BRANCH_NAME"
+    echo "We accept multiple commits as soon as they are commas-separated."
+    echo "e.g: ./contrib/backport_to_stable_branch.sh stable-2.2 6892670d317698771be7e96ce9032bc27d3fd1e5,8756c553cc8c213fc4996fc5202c7b687eb645a3 my-work"
+    exit 1
+  fi
+}
+
+
+########
+# MAIN #
+########
+test_args "$@"
+git_status
+verify_commit
+checkout
+cherry_pick
+push
+cleanup