cleanup: make an empty commit for actions/checkout

Make an empty detached commit for actions/checkout to properly
clean the project's workspace before checking out new code
revision.

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

cleanup/README.md

@@ -1,14 +1,52 @@
 # Setup environment
 
-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). 
-When submodules are changed, the standard checkout fails with git errors. 
-It's a well-known problem and the related issue 
-[actions/checkout#418](https://github.com/actions/checkout/issues/418) is still opened.
+Action creates an empty detached commit, which helps subsequent
+[actions/checkout](https://github.com/actions/checkout) action
+correctly clean the workspace.
 
-## How to use GitHub Action from GitHub workflow
+## Usage
 
-Add the following code to the running steps before `checkout` action:
+Add the following line to your workflow before the `actions/checkout` action:
+```diff
++ - uses: tarantool/actions/cleanup@master
+  - uses: actions/checkout@v3
+    ...
 ```
-  - uses: tarantool/actions/cleanup@master
+
+## Explanation and rationale
+
+The main reason to add
+this action is [tarantool/tarantool-qa#145](https://github.com/tarantool/tarantool-qa/issues/145). 
+When submodules change, actions/checkout fails 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 doesn't 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.

cleanup/action.yml

@@ -1,11 +1,14 @@
 ---
 name: 'Clean workspace'
-description: 'Clean workspace directory after previous workflow run'
+description: 'Clean workspace directory by making an empty detached commit'
 runs:
   using: 'composite'
   steps:
     - run: |
         set -e
         shopt -s dotglob
-        rm -rf ${{ github.workspace }}/*
+        git checkout -f \
+          $(git -c user.name=TarantoolBot -c [email protected] commit-tree \
+            $(git hash-object -t tree /dev/null) \
+          < /dev/null) || :
       shell: bash