Fix git diff command in CI script

[alfredxing]

Addresses #2615 (comment).

Found this neat command that seems to work: git diff --name-only ..master. It shows all files changed in that pull request only (similar to the "Files Changed" tab on GitHub). I tested this on a few open PR's in this repo.

If it works well in script/proof we can consider using it in script/test as well.

[parkr]

Can you modify a site file in this PR then revert that change and squash? That interim commit should trigger the proof if all is well.

[ivantsepp]

I'm not sure if that works? I tried it with #2007.

I've always used git diff --name-only $(git merge-base master HEAD) to mimic the "Files Changed" tab on Github.

Edit: Actually git diff --name-only master... seems to work as well

[alfredxing]

Worked as expected: https://travis-ci.org/jekyll/jekyll/jobs/31436061#L126 (commit above disappeared due to squash).