When creating a PR on a PR, the diff is wrong.

[drortirosh]

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1. I created a PR on to add a new file into the project.
2. Another PR was created against that new file, targeted to merge into the first PR
3. will looking at the 2nd PR, the diff is not against the 1st PR but against the "master" branch.
4. it is possible to select the specific commits from 2nd PR, but it is annoying, as it was clearly created against 1st PR, not master.