Here's the relevant API page:
https://developer.github.com/v3/pulls/#merge-a-pull-request-merge-button

We'll need to add:
'merge_method': 'rebase'

Here:
https://cs.chromium.org/chromium/src/third_party/WebKit/Tools/Scripts/webkitpy/w3c/wpt_github.py?q=webkitpy/w3c+merge+.py&sq=package:chromium&dr=CSs&l=90

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e0a2ec09d1087b572e70002fce286231c0fef823

commit e0a2ec09d1087b572e70002fce286231c0fef823
Author: jeffcarp <jeffcarp@chromium.org>
Date: Thu Jan 26 05:51:36 2017

[WPT Export] Add merge_method: rebase to WPTGitHub.merge_pull_request

BUG= 682883 

Review-Url: https://codereview.chromium.org/2658713002
Cr-Commit-Position: refs/heads/master@{#446244}

[modify] https://crrev.com/e0a2ec09d1087b572e70002fce286231c0fef823/third_party/WebKit/Tools/Scripts/webkitpy/w3c/wpt_github.py

It looks like the script is still creating merge commits:
https://github.com/w3c/web-platform-tests/commits/master

 Issue 693351  has been merged into this issue.

Turns out this isn't working because the rebase merge method for PRs is still in preview mode.
https://developer.github.com/v3/pulls/#input-2

So we can either:
- Opt into the preview API now using the media Accept header
- Wait until it becomes stable

How about we just wait then, and letting it be consistent across the quarter should make it easier to track the "time to export" metrics.

This was enabled by GitHub recently and commits are now being rebased and fast forwarded onto master: 
https://github.com/w3c/web-platform-tests/commits/master?after=690108a07534ca09de27af8509a6bcc07ac0969e+5