RF+ENH: add testing that step by step is identical to monolythic

Ad-hoc -- use "shell" as interpreter for ``` whenever it should not become
part of the "step by step" script

.github/workflows/tests.yaml

@@ -29,3 +29,6 @@ jobs:
 
     - name: Test example in the README.md
       run: bash <(sed -n -e '/^ *#!/,/^```$/p' README.md | grep -v '```')
+
+    - name: Test that the monolythic example is identical to the step by step
+      run: scripts/tests/test_README_versions

README.md

@@ -215,7 +215,7 @@ datalad install -d . -s ///repronim/containers code/containers
 
 Now let's take a look at what we have.
 
-```bash
+```shell
 /ds000003-qc # The root dataset contains everything
  |--/sourcedata # we call it source, but it is actually ds000003-demo
  |--/code/containers # repronim/containers, this is where our non-custom code lives
@@ -246,7 +246,7 @@ datalad run -m "Downgrade/Freeze mriqc container version" \
 
 **Option 2: ///repronim/containers**
 
-```bash
+```shell
 # Run from ~/my-experiments/ds000003-qc/containers
 datalad run -m "Downgrade/Freeze mriqc container version" \
     code/containers/scripts/freeze_versions bids-mriqc=0.16.0

scripts/tests/test_README_versions

@@ -0,0 +1,3 @@
+#!/bin/bash
+
+diff -Naur <(sed -n -e '/^ *#!/,/^```$/p' README.md | grep -v -E '^([()#]|PS4|cd .*mktemp|```)') <(sed -n -e '/Walkthrough/,/Do not change lines below/p' README.md | sed -n -e '/```bash/,/```/p' | grep -v -e '```' -e '~/my-experiments')