Monthly cron job checking for submodule updates (#1741)

* Create a script that generates `diff.log` if git submodules have tags more recent that the commit they're currently at.
* A Github Action runs this script monthly.
* _If_ updates found, Github Action creates an Issue like so: https://github.com/brenthuisman/arbor/issues/9
* Actually updating the git submodule and Spack file are still manual

First step in addressing #1731

.github/workflows/check-submodules.yml

+name: Update submodules
+
+on:
+  schedule:
+    # - cron: '0 2 * * 0' # run at 2 AM every sunday
+    - cron: '5 4 2 * *' # 04:05 every 2nd of month
+  push:
+    branches: [ 'ci/auto-update-submodules' ]
+    
+jobs:
+  update_submodules:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Clone w/ submodules
+        uses: actions/checkout@v2
+        with:
+          submodules: recursive
+          fetch-depth: 0
+      - name: Check submodules for updates
+        run: scripts/submodule-diff.sh
+      - name: Create Issue From File
+        uses: peter-evans/create-issue-from-file@v3
+        with:
+          title: '[AUTOMATED] Git submodule updates found'
+          content-filepath: ./diff.log
+          labels: |
+            dependency update
+          assignees: brenthuisman

.gitmodules

 [submodule "google-benchmark"]
 	path = ext/google-benchmark
 	url = https://github.com/google/benchmark
+	branch = main
 [submodule "python/pybind11"]
 	path = python/pybind11
 	url = https://github.com/pybind/pybind11.git
+	branch = master
 [submodule "ext/fmt"]
 	path = ext/fmt
 	url = https://github.com/fmtlib/fmt.git
+	branch = master

scripts/submodule-diff.sh

+#!/usr/bin/env bash
+# Check any git submodule remotes for updates, and print difference with current Arbor repo state to `diff.log`
+
+git submodule foreach 'git describe HEAD --tags' | tee current_state_of_git_submodules_in_arbor_repo.log
+git submodule foreach 'git fetch'
+git submodule foreach 'git describe `git log --branches -1 --pretty=format:"%H"` --tags --abbrev=0' | tee upstream_state_of_git_submodules.log
+if diff current_state_of_git_submodules_in_arbor_repo.log upstream_state_of_git_submodules.log ; then
+    echo "No submodule updates found"
+else
+    diff current_state_of_git_submodules_in_arbor_repo.log upstream_state_of_git_submodules.log -U 10000 > diff.log
+fi
+rm current_state_of_git_submodules_in_arbor_repo.log
+rm upstream_state_of_git_submodules.log