Use different git diff command.

Closes #3401988

.gitlab-ci.yml

@@ -46,7 +46,7 @@ variables:
   # Let composer know what self.version means.
   COMPOSER_ROOT_VERSION: "${CI_MERGE_REQUEST_TARGET_BRANCH_NAME}${CI_COMMIT_BRANCH}-dev"
   CONCURRENCY: 24
-  GIT_DEPTH: "3"
+  GIT_DEPTH: "50"
   PARENT_PIPELINE_ID: $CI_PIPELINE_ID
   _TARGET_PHP: "8.2"
 
@@ -413,5 +413,4 @@ default:
   script:
     - export TARGET_BRANCH=${CI_MERGE_REQUEST_TARGET_BRANCH_NAME}${CI_COMMIT_BRANCH}
     - git fetch -vn --depth=$GIT_DEPTH origin "+refs/heads/$TARGET_BRANCH:refs/heads/$TARGET_BRANCH"
-    - export MODIFIED=`git diff --name-only refs/heads/$TARGET_BRANCH|while read r;do echo "$CI_PROJECT_DIR/$r";done|tr "\n" " "`
-    - echo $MODIFIED | tr ' ' '\n' | yarn --cwd=./core run -s spellcheck:core --no-must-find-files --file-list stdin
+    - git diff $CI_MERGE_REQUEST_DIFF_BASE_SHA --name-only | sed "s_^_../_" | yarn --cwd=./core run -s spellcheck:core --no-must-find-files --file-list stdin