prepare-workspace: make actions/checkout more reliable

Make an empty detached commit for actions/checkout to properly
clean the project's workspace before checking out new code
revision. It makes the actions/checkout resilient to
various problems related to git submodules.

Part of #28
Related to tarantool/tarantool-qa#145

cleanup/README.md

@@ -1,4 +1,4 @@
-# Setup environment
+# Clean up the workspace
 
 Action cleans workspace directory after previous run. The main reason to add
 this action is [tarantool/tarantool-qa#145](https://github.com/tarantool/tarantool-qa/issues/145). 

prepare-checkout/README.md

@@ -0,0 +1,57 @@
+# Prepare workspace for checkout action
+
+This action creates and checks out an empty detached commit.
+It helps subsequent [`actions/checkout`](https://github.com/actions/checkout) action
+correctly clean the workspace.
+
+## Usage
+
+Add the following line to your workflow before the `actions/checkout` action:
+
+```diff
++ - uses: tarantool/actions/prepare-checkout@master
+  - uses: actions/checkout@v3
+    ...
+```
+
+## Explanation and rationale
+
+This action is a solution for 
+[tarantool/tarantool-qa#145](https://github.com/tarantool/tarantool-qa/issues/145).
+First attempt to solve this was the `tarantool/actions/cleanup` action.
+The `tarantool/actions/prepare-checkout` action is a softer alternative to it.
+
+When submodules change, `actions/checkout` can fail with git errors. 
+It's a well-known problem and the related issues are still open:
+actions/checkout#354,
+actions/checkout#385,
+actions/checkout#418, and
+actions/checkout#590.
+
+Before checking out a new revision, `actions/checkout` runs the following code:
+
+```bash
+# removes ignored and non-versioned files
+git clean -ffdx
+# resets workspace to the commit, on which it was left
+# after the last job run
+git reset --hard
+```
+
+The problem is that when a workflow fails because of a particular commit,
+the repository still stays on that commit. On the next job run, 
+`actions/checkout` will run the above code, restore that particular commit,
+and fail to make a proper code checkout.
+
+By creating a detached empty commit, this actions forces `actions/checkout`
+to clean up the project's workspace entirely, removing any files that could
+break checkout. Meanwhile, the `.git` directory stays intact, so full checkout
+isn't required and the workflow does not waste much time.
+
+If this is a first job run on a particular runner and there's no repository yet,
+the command in this action will silently fail, thanks to `|| :`,
+but the action itself will succeed.
+
+This action uses a solution proposed in a
+[comment](https://github.com/actions/checkout/issues/590#issuecomment-970586842)
+at actions/checkout#590.

prepare-checkout/action.yml

@@ -0,0 +1,13 @@
+---
+name: 'Prepare workspace for checkout'
+description: 'Prepare workspace for the actions/checkout action'
+runs:
+  using: 'composite'
+  steps:
+    - run: |
+        set -e
+        git checkout -f \
+          $(git -c user.name=TarantoolBot -c [email protected] \
+          commit-tree $(git hash-object -t tree /dev/null) -m 'Empty commit' \
+          < /dev/null) || :
+      shell: bash