Make auto-merge more conservative

Only do auto-merges when the pull-request contains exactly one commit.
This prevents pull-requests which are manually amended from being merged
automatically.

Author: thomas-zahner

.github/workflows/auto-merge.yml

@@ -8,7 +8,11 @@ permissions:
 jobs:
   auto-merge:
     runs-on: ubuntu-latest
-    if: github.event.pull_request.user.login == 'dependabot[bot]' && github.repository == 'lycheeverse/lychee'
+    if: |
+      github.event.pull_request.commits == 1 &&
+      github.repository == 'lycheeverse/lychee' &&
+      github.event.pull_request.user.login == 'dependabot[bot]'
+
     steps:
       - name: Enable auto-merge for Dependabot PRs
         # Merge all updates as long as they pass CI