Only spellcheck added and modified files

Author: m-kuhn

ci/travis/check_spelling.sh

@@ -9,7 +9,7 @@ echo "Spell check"
 if [[ !  -z  $TRAVIS_PULL_REQUEST_BRANCH  ]]; then
   # if on a PR, just analyse the changed files
   echo "TRAVIS PR BRANCH: $TRAVIS_PULL_REQUEST_BRANCH"
-  FILES=$(git diff --name-only HEAD $(git merge-base HEAD master) | tr '\n' ' ' ) 
+  FILES=$(git diff --diff-filter=AM --name-only $(git merge-base HEAD master) | tr '\n' ' ' )
 fi
 
 ./scripts/chkspelling_ag.sh $FILES