ci: improve git diff, avoid gitlab bug on CI_MERGE_REQUEST_DIFF_BASE_SHA

.gitlab-ci.yml

@@ -3,12 +3,12 @@ workflow:
     # Disable those non-protected push triggered pipelines
     - if: '$CI_COMMIT_REF_NAME != "master" && $CI_COMMIT_BRANCH !~ /^release\/v/ && $CI_COMMIT_TAG !~ /^v\d+\.\d+(\.\d+)?($|-)/ && $CI_COMMIT_TAG !~ /^qa-test/ && $CI_PIPELINE_SOURCE == "push"'
       when: never
-    # when running merged result pipelines, CI_COMMIT_SHA represents the temp commit it created.
-    # Please use PIPELINE_COMMIT_SHA at all places that require a commit sha of the original commit.
-    - if: $CI_OPEN_MERGE_REQUESTS != null
+    # merged result pipelines
+    - if: $CI_MERGE_REQUEST_SOURCE_BRANCH_SHA
       variables:
         PIPELINE_COMMIT_SHA: $CI_MERGE_REQUEST_SOURCE_BRANCH_SHA
-    - if: $CI_OPEN_MERGE_REQUESTS == null
+    # else
+    - if: $CI_MERGE_REQUEST_SOURCE_BRANCH_SHA == null || $CI_MERGE_REQUEST_SOURCE_BRANCH_SHA == ""
       variables:
         PIPELINE_COMMIT_SHA: $CI_COMMIT_SHA
     - when: always

.gitlab/ci/common.yml

@@ -334,9 +334,10 @@ variables:
       TEMP_FILE=$(mktemp)
       # merged results pipelines, by default
       if [[ -n $CI_MERGE_REQUEST_SOURCE_BRANCH_SHA ]]; then
-        git fetch origin $CI_MERGE_REQUEST_DIFF_BASE_SHA --depth=1 ${GIT_FETCH_EXTRA_FLAGS}
-        git fetch origin $CI_MERGE_REQUEST_SOURCE_BRANCH_SHA --depth=1 ${GIT_FETCH_EXTRA_FLAGS}
-        git diff --name-only $CI_MERGE_REQUEST_DIFF_BASE_SHA $CI_MERGE_REQUEST_SOURCE_BRANCH_SHA > "$TEMP_FILE"
+        git fetch origin $CI_MERGE_REQUEST_TARGET_BRANCH_SHA
+        git fetch origin $CI_MERGE_REQUEST_SOURCE_BRANCH_SHA
+
+        git diff --name-only $CI_MERGE_REQUEST_TARGET_BRANCH_SHA...$CI_MERGE_REQUEST_SOURCE_BRANCH_SHA > "$TEMP_FILE"
         GIT_DIFF_OUTPUT=$(cat "$TEMP_FILE")
         git fetch origin $CI_COMMIT_SHA --depth=1 ${GIT_FETCH_EXTRA_FLAGS}
       # merge request pipelines, when the mr got conflicts