[BUG] CI is failing due to wrong diff comparison in Danger

[dkundel]

Description

Right now the CI is failing on basically every run because it complains that people removed pixels from others. It appears to be because it's using the wrong diff for the check.

Expected Behavior

PRs like this one should succeed: #2045

Actual Behavior

PR failed because it's claiming it removed additional pixels https://github.com/twilio-labs/open-pixel-art/pull/2045/checks?check_run_id=589878945

Possible Fix

The diff that's being generated by Danger seems to be wrong. I did some digging but couldn't figure out yet how to get the merge commit instead. I asked on Spectrum but haven't heard back yet. Next step would probably be to report a bug on danger/danger-js.

Steps to Reproduce

1. Check the changes in this PR: https://github.com/twilio-labs/open-pixel-art/pull/2045/files
2. There is clearly only one line added
3. Check the Danger logs of https://github.com/twilio-labs/open-pixel-art/pull/2045/checks?check_run_id=589878945
4. It says it failed because pixels were removed.

Context

This is currently blocking the automatic PR experience for users.