From b881f32687 (travis/changed_templates: filter by git itself, 2020-05-23), we replaced git-diff(1) with git-diff-tree(1). The change wasn't equivalence, though. We used to compare between merge-base of FETCH_HEAD and HEAD. From that commit, we started to compare FETCH_HEAD and HEAD instead. Fix them by changing left tree-ish to the merge-base.