ci: do not show an error if branch contains one commit only (#130)

... as we squash all commits and use the PR as commit message it makes
no sense to check for single commits in a branch and report an error.
Would be true only if the PR content wouldn't be used as commit message.
GitHub uses the original commit message which might differ from the PR
title.

.github/workflows/pull-request.yml

@@ -0,0 +1,50 @@
+---
+name: "Pull Request"
+
+# yamllint disable-line rule:truthy
+on:
+  pull_request_target:
+    types:
+      - opened
+      - edited
+      - synchronize
+    branches-ignore:
+      - "release-please--branches--*"
+
+jobs:
+  main:
+    name: Validate PR title
+    runs-on: ubuntu-latest
+    steps:
+      # yamllint disable-line rule:comments
+      - uses: amannn/action-semantic-pull-request@e9fabac35e210fea40ca5b14c0da95a099eff26f # v5.4.0
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          types: |
+            build
+            chore
+            ci
+            docs
+            feat
+            fix
+            perf
+            refactor
+            revert
+            style
+            test
+          # Configure which scopes are allowed.
+          # deps - dependency updates
+          # main - for release-please (scope used for releases)
+          scopes: |
+            deps
+            main
+          requireScope: false
+          subjectPattern: ^(?![A-Z]).+$
+          subjectPatternError: |
+            The subject "{subject}" found in the pull request title "{title}"
+            didn't match the configured pattern. Please ensure that the subject
+            doesn't start with an uppercase character.
+          wip: true
+          validateSingleCommit: false
+          validateSingleCommitMatchesPrTitle: false