Bug 243277

EWS is using the base.sha received from the Github pull request hook to determine if a PR branch needs to be rebased before building.

As discussed here - https://github.com/github/docs/issues/431 - base.sha is the sha for the current HEAD of the branch we want to merge in to (main), which may not be the commit that the PR branch is based on (the nearest common ancestor of head:MAIN and the PR branch), especially if `--no-rebase` was used for `git webkit pr`.

I think this means we effectively never rebase PRs before building and running tests, except we still update when running 'compile-webkit-without-change', so the test results comparison is invalid.

An example is here:

Build log: https://ews-build.webkit.org/#/builders/64/builds/26323

This checked out the current branch, with hash 1f7bd0da - https://github.com/WebKit/WebKit/commit/1f7bd0dadc55fd7a00223ede7600c26cc2eef517. The parent of that commit is https://github.com/WebKit/WebKit/commit/af751da03e1659f63ecddb9cf8eb7c8cfaa69520 - 252657@main.

We then ran tests, failed and built webkit without the changes - https://ews-build.webkit.org/#/builders/68/builds/21414

This checked out 659f5b107515cfac3610d557b4c38a2ec0f00b57, which is 252808@main.

So the original tests ran with 252657@main (plus my changes), the comparison ran with 252808@main.


I think we need to switch to using `git merge-base --is-ancestor main ${pr-branch}` to determine if the PR branch is rebased or not, rather than using base.sha from GitHub.