move checkout action to separate path

checkout/action.yml

@@ -0,0 +1,22 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+name: 'Checkout git repository'
+author: 'Manuel Friedli'
+description: |
+  Checkout a git repository.
+
+  It performs a shallow checkout and does not require NodeJS to run.
+
+inputs:
+  hostname:
+    description: The hostname of the forge we're checking out from.
+    default: "gittr.ch"
+runs:
+  using: "composite"
+  steps:
+    - run: |
+        set -eu
+        git init --quiet --initial-branch main
+        git remote add origin "https://${{ forge.token }}@${{ inputs.hostname }}/${{ forge.repository }}.git"
+        git fetch --quiet --no-tags --depth=1 origin "${{ forge.sha }}"
+        git checkout --quiet "${{ forge.sha }}"
+      shell: bash