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.
- Use plumbing git-diff-tree(1) instead of porcelain git-diff(1)
- Disable rename detection, gains some speed up and fix missing template
with name-status Renamed and Copied.
- Use "--diff-filter" to filter by Git itself.
- Tell Git to only check 'srcpkgs/*/template', this glob will only look
into template file inside direct sub-directory of srcpkgs
- With all of those options, we don't need a grep and a pipe
