roll_merge.py manually checks out V8 into a temporary directory, locally
builds a cherrypick, and uploads this to Gerrit. However, Gerrit has its
own REST API which allows cherrypicking. Using this API directly has two
advantages:

  1) We don't need to perform any local checkouts, so it's much faster,
     and
  2) The cherry-picked commit is marked as a cherry-pick by Gerrit,
     which means Rubber-Stamper-Bot will treat it as a cherry-pick.

The implementation for now is very simple, and doesn't support things
like cherry-picking multiple revisions or applying an additional local
patch. It does, however, increment the patch value in v8-version.h, and
tries to set Owners-Override +1.

Bug: v8:12849
Change-Id: Ie242dbec6b3d24f5118d601e9d326465d190a8f0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3644609
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81072}