CI limit diff to commit range in flake8_diff.sh (#8097)

build_tools/travis/flake8_diff.sh

@@ -128,7 +128,7 @@ check_files() {
     options="$2"
     # Conservative approach: diff without context (--unified=0) so that code
     # that was not changed does not create failures
-    git diff --unified=0 $COMMIT -- $files | flake8 --diff --show-source $options
+    git diff --unified=0 $COMMIT_RANGE -- $files | flake8 --diff --show-source $options
 }
 
 if [[ "$MODIFIED_FILES" == "no_match" ]]; then