Improve commit retrieval

.github/workflows/linter.yml

@@ -14,8 +14,29 @@ jobs:
     name: RuboCop
     runs-on: ubuntu-latest
     steps:
-      - name: Checkout sources
-        uses: actions/checkout@v4
+      - name: Determine target branch
+        run: |
+          target_branch=${{ github.base_ref || 'mampf-next' }}
+          echo "Target branch is: $target_branch"
+          echo "TARGET_BRANCH=$target_branch" >> $GITHUB_ENV
+
+      # for the " + 1", see:
+      # https://github.com/actions/checkout/issues/552#issuecomment-1167086216 
+      - name: Calculate fetch depth
+        run: |
+          depth=$(( ${{ github.event.pull_request.commits }} + 1 ))
+          echo "Fetch depth is: $depth"
+          echo "PR_FETCH_DEPTH=$depth" >> $GITHUB_ENV
+      
+      - name: 'Checkout PR branch and all PR commits'
+        uses: actions/checkout@v3
+        with:
+          ref: ${{ github.event.pull_request.head.ref }}
+          fetch-depth: ${{ env.PR_FETCH_DEPTH }}
+
+      - name: Fetch target branch with enough history for a common merge-base commit
+        run: |
+          git fetch origin ${{ github.event.pull_request.base.ref }}
 
       - name: Set up Ruby 3
         uses: ruby/setup-ruby@v1
@@ -28,20 +49,22 @@ jobs:
       # But in the end, one has to open a PR anyways and then the correct target
       # branch is identified.
       # TODO: rename to 'dev' when 'mampf-next' branch is renamed.
-      - name: Fetch target branch
-        run: |
-          targetBranch=${{ github.base_ref || 'mampf-next' }}
-          echo "Target branch is: $targetBranch"
-          git fetch origin $targetBranch:$targetBranch --depth=1
+      # - name: Fetch target branch
+      #   run: |
+      #     targetBranch=${{ github.base_ref || 'mampf-next' }}
+      #     echo "Target branch is: $targetBranch"
+      #     git fetch origin $targetBranch:$targetBranch --depth=1
 
       # adapted from https://github.com/actions/checkout/issues/520#issuecomment-1167205721
       # and: https://robertfaldo.medium.com/commands-to-run-rubocop-and-specs-you-changed-in-your-branch-e6d2f2e4110b
       # --diff-filter=d excludes deleted files
       # "|| test $? = 1;" is used to ignore the exit code of grep when no files
       # are found matching the pattern
+      # For the "three dots" ... syntax, see
+      # https://community.atlassian.com/t5/Bitbucket-questions/Git-diff-show-different-files-than-PR-Pull-Request/qaq-p/2331786
       - name: Get changed ruby files (git diff)
         run: |
-          changedFiles=$(git diff --name-only --diff-filter=d -r $targetBranch | grep '\.rb$' || test $? = 1;)
+          changedFiles=$(git diff --name-only --diff-filter=d -r $targetBranch...HEAD | grep '\.rb$' || test $? = 1;)
           printf "Changed ruby files: \n$changedFiles"
           echo "CHANGED_FILES=\"$changedFiles\"" >> $GITHUB_ENV