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.
f5417b7b99