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/squash-branches.sh

@@ -0,0 +1,69 @@
+#!/bin/bash
+
+if [[ $# -lt 3 || "$1" == "--help" || "$1" = "-h" ]]
+then
+  name=$( basename $0 )
+  cat <<- USAGE
+  Usage: $name workbranch base/branch user/branch [user/branch [...]]
+  
+  * workbranch will be created locally from base/branch
+  * base/branch and user/branch may be specified as user:branch to allow
+    easy copying from Github PRs
+  * Remotes for each user must already be set up
+USAGE
+exit 0
+fi
+
+work="$1"
+shift
+
+branches=( $( echo "${@}" | sed "s/:/\//" ) )
+base="${branches[0]}"
+unset branches[0]
+
+set -e
+
+users=()
+for b in "${branches[@]}"
+do
+  users+=( $( echo $b | cut -d/ -f1 ) )
+done
+
+users=( $( printf '%s\n' "${users[@]}" | sort -u ) )
+
+git fetch --multiple upstreams "${users[@]}"
+git checkout -B "$work" --no-track "$base"
+
+for b in "${branches[@]}"
+do
+  git merge --squash "${b}"
+  git commit -S # Use the commit message provided on the PR
+done
+
+# Make sure the commits look right
+git log --show-signature "$base..HEAD"
+
+parts=( $( echo $base | sed "s/\// /" ) )
+repo="${parts[0]}"
+b="${parts[1]}"
+push=$repo
+if [[ "$push" == "upstream" ]]
+then
+  push="upstream-push"
+fi
+if [[ "$repo" == "upstream" ]]
+then
+  repo="upstreams"
+fi
+cat << PUSH
+
+-------------------------------------------------------------------
+This script will not push. Verify everything is correct, then push
+to your repo, and create a PR if necessary. Once the PR is approved,
+run:
+
+git push $push HEAD:$b
+git fetch $repo
+-------------------------------------------------------------------
+PUSH
+