ci: use different base commit for merge result pipelines

.gitlab/ci/pre_check.yml

@@ -183,12 +183,24 @@ baseline_manifest_sha:
   tags: [fast_run, shiny]
   script:
     - |
-      if [ -n "$CI_MERGE_REQUEST_DIFF_BASE_SHA" ]; then
+      # merged results pipelines, by default
+      # diff between target-branch-head and merged-result-head
+      if [ -n "$CI_MERGE_REQUEST_TARGET_BRANCH_SHA" ]; then
+        git fetch origin $CI_MERGE_REQUEST_TARGET_BRANCH_SHA --depth=1
+        git checkout FETCH_HEAD
+        idf-build-apps dump-manifest-sha \
+          --manifest-files $(find . -name ".build-test-rules.yml" | xargs) \
+          --output .manifest_sha
+      # merge request pipelines, when the mr got conflicts
+      # diff between diff-base-sha and merge-request-head
+      elif [ -n "$CI_MERGE_REQUEST_DIFF_BASE_SHA" ]; then
         git fetch origin $CI_MERGE_REQUEST_DIFF_BASE_SHA --depth=1
         git checkout FETCH_HEAD
         idf-build-apps dump-manifest-sha \
           --manifest-files $(find . -name ".build-test-rules.yml" | xargs) \
           --output .manifest_sha
+      # other pipelines, like the protected branches pipelines
+      # not triggered in this job
       fi
   artifacts:
     paths: