Reland "[wpt-importer] Write new bugs to `TestExpectations` (11/n)"

This is a reland of commit b5741ab6

Changes from original:
  Don't dirty the checkout if no lines were updated. This then skips
  creating the bug fixup CL.

Original change's description:
> [wpt-importer] Write new bugs to `TestExpectations` (11/n)
>
> The importer creates an ancestor CL to update expectations with the
> corresponding bugs filed.
>
> Bug: 40631540
> Change-Id: I9e3cfe9885a2f10bbe8674b4282cf8ec6f53aac5
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5389541


> Reviewed-by: Weizhong Xia <weizhong@google.com>
> Commit-Queue: Jonathan Lee <jonathanjlee@google.com>
> Cr-Commit-Position: refs/heads/main@{#1278449}

Bug: 40631540
Change-Id: I4809e7c52a9f87c6932ecef21ce3d2f0aa876128
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5402851


Commit-Queue: Jonathan Lee <jonathanjlee@google.com>
Reviewed-by: Weizhong Xia <weizhong@google.com>
Cr-Commit-Position: refs/heads/main@{#1279306}