ci: fix changelog step on shallow checkout and small repos

- Set fetch-depth: 0 so previous tag lookups work across full history.
- Use `-n 20` instead of HEAD~20..HEAD, which fails when the repo has
  fewer than 20 commits (e.g. on the first release).

.gitea/workflows/release.yml

@@ -15,6 +15,8 @@ jobs:
     steps:
       - name: Checkout repo
         uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
 
       - name: Extract version from tag
         id: version
@@ -69,7 +71,7 @@ jobs:
         run: |
           PREV_TAG=$(git tag --sort=-v:refname | head -2 | tail -1)
           if [ -z "$PREV_TAG" ] || [ "$PREV_TAG" = "${{ gitea.ref_name }}" ]; then
-            CHANGELOG=$(git log --oneline --no-decorate HEAD~20..HEAD)
+            CHANGELOG=$(git log --oneline --no-decorate -n 20)
           else
             CHANGELOG=$(git log --oneline --no-decorate ${PREV_TAG}..HEAD)
           fi