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

.github/workflows/missing-commits.yml

@@ -0,0 +1,60 @@
+name: missing-commits
+
+on:
+  push:
+    branches:
+      # Only check that the branches are up to date when updating the
+      # relevant branches.
+      - develop
+      - release
+
+jobs:
+  check:
+    runs-on: ubuntu-24.04
+    steps:
+    - uses: actions/checkout@v4
+      with:
+        fetch-depth: 0
+    - name: Check for missing commits
+      id: commits
+      env:
+        SUGGESTION: |
+
+          If you are reading this, then the commits indicated above are
+          missing from "develop" and/or "release". Do a reverse-merge
+          as soon as possible. See CONTRIBUTING.md for instructions.
+      run: |
+        set -o pipefail
+        # Branches ordered by how "canonical" they are. Every commit in
+        # one branch should be in all the branches behind it
+        order=( master release develop )
+        branches=()
+        for branch in "${order[@]}"
+        do
+          # Check that the branches exist so that this job will work on
+          # forked repos, which don't necessarily have master and
+          # release branches.
+          if git ls-remote --exit-code --heads origin \
+            refs/heads/${branch} > /dev/null
+          then
+            branches+=( origin/${branch} )
+          fi
+        done
+
+        prior=()
+        for branch in "${branches[@]}"
+        do
+          if [[ ${#prior[@]} -ne 0 ]]
+          then
+            echo "Checking ${prior[@]} for commits missing from ${branch}"
+            git log --oneline --no-merges "${prior[@]}" \
+              ^$branch | tee -a "missing-commits.txt"
+            echo
+          fi
+          prior+=( "${branch}" )
+        done
+        if [[ $( cat missing-commits.txt | wc -l ) -ne 0 ]]
+        then
+          echo "${SUGGESTION}"
+          exit 1
+        fi