Hey, just successfully implemented a workaround which makes use of the API:
LAST_SUCCESSFUL_COMMIT=`curl -Ss -u "$API_KEY:" $LAST_SUCCESSFUL_BUILD_URL | jq -r '.["vcs_revision"]'`
CHANGES_SINCE_LAST_COMMIT=`git diff --name-only $CIRCLE_SHA1..$LAST_SUCCESSFUL_COMMIT`
changesArray=( $CHANGES_SINCE_LAST_COMMIT )
You need to generate an API key which could be defined via environment variables.
Downside is that the algorithm only works for the latest build. In case you restart an older one, the change set will be empty.