Update branch management and merge / release processes (#5215)

* Has more steps, but allows merges to develop to continue when a
  beta / RC is pending, increasing developer velocity.
* Add a CI job to check that no reverse merges have been missed.
* Add some useful scripts in bin/git:
  * Set up upstreams as expected for safer pushes
  * Squash a bunch of branches
  * Set the version number

bin/git/update-version.sh

@@ -0,0 +1,58 @@
+#!/bin/bash
+
+if [[ $# -ne 3 || "$1" == "--help" || "$1" = "-h" ]]
+then
+  name=$( basename $0 )
+  cat <<- USAGE
+  Usage: $name workbranch base/branch version
+
+  * workbranch will be created locally from base/branch. If it exists,
+    it will be reused, so make sure you don't overwrite any work.
+  * base/branch may be specified as user:branch to allow easy copying
+    from Github PRs.
+USAGE
+exit 0
+fi
+
+work="$1"
+shift
+
+base=$( echo "$1" | sed "s/:/\//" )
+shift
+
+version=$1
+shift
+
+set -e
+
+git fetch upstreams
+
+git checkout -B "${work}" --no-track "${base}"
+
+push=$( git rev-parse --abbrev-ref --symbolic-full-name '@{push}' \
+              2>/dev/null ) || true
+if [[ "${push}" != "" ]]
+then
+  echo "Warning: ${push} may already exist."
+fi
+
+build=$( find -name BuildInfo.cpp )
+sed 's/\(^.*versionString =\).*$/\1 "'${version}'"/' ${build} > version.cpp && \
+diff "${build}" version.cpp && exit 1 || \
+mv -vi version.cpp ${build}
+
+git diff
+
+git add ${build}
+
+git commit -S -m "Set version to ${version}"
+
+git log --oneline --first-parent ${base}^..
+
+cat << PUSH
+
+-------------------------------------------------------------------
+This script will not push. Verify everything is correct, then push
+to your repo, and create a PR as described in CONTRIBUTING.md.
+-------------------------------------------------------------------
+PUSH