ci(shelllint): align PR linting with local ShellCheck runs

Lint only changed shell files in pull requests, keep workflow steps in sh,
and report ShellCheck findings like local runs while pinning version 0.9.0-1.

Signed-off-by: Srikanth Muppandam <smuppand@qti.qualcomm.com>

Author: smuppand

.github/workflows/shellcheck.yml

@@ -11,29 +11,44 @@ jobs:
   shellcheck:
     runs-on: ubuntu-latest
 
+    defaults:
+      run:
+        shell: sh
+
     steps:
       - name: Checkout source
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
 
       - name: Install ShellCheck from apt
         run: |
           sudo apt-get update
           sudo apt-get install -y shellcheck=0.9.0-1
+          shellcheck --version
 
       - name: Run ShellCheck on changed .sh files in PR
         if: github.event_name == 'pull_request'
         run: |
-          echo "Checking only changed shell files in PR..."
-          git fetch origin ${{ github.base_ref }}
-          FILES=$(git diff --diff-filter=d --name-only origin/${{ github.base_ref }} -- '*.sh')
+          set -eu
+
+          BASE_BRANCH="${{ github.base_ref }}"
+          git fetch origin "${BASE_BRANCH}" --depth=1
+
+          MERGE_BASE="$(git merge-base HEAD "origin/${BASE_BRANCH}")"
+          FILES="$(git diff --diff-filter=d --name-only "${MERGE_BASE}"...HEAD -- '*.sh')"
+
           if [ -n "$FILES" ]; then
-            echo "$FILES" | tr '\n' '\0' | xargs -0 -r shellcheck -S warning -e SC1091,SC2230,SC3043
+            echo "Checking changed shell files:"
+            printf '%s\n' "$FILES"
+            printf '%s\n' "$FILES" | tr '\n' '\0' | xargs -0 -r shellcheck -s sh -e SC1091,SC2230,SC3043
           else
             echo "No shell files to lint."
           fi
 
-      - name: Run ShellCheck on all .sh files (main or manual trigger)
+      - name: Run ShellCheck on all .sh files (push/manual)
         if: github.event_name != 'pull_request'
         run: |
+          set -eu
           echo "Linting all shell files in repository..."
-          find . -type f -name '*.sh' -print0 | xargs -0 -r shellcheck -S warning -e SC1091,SC2230,SC3043
+          find . -type f -name '*.sh' -print0 | xargs -0 -r shellcheck -s sh -e SC1091,SC2230,SC3043