crosvm: Script for rebasing on main from chromeos branch.

chromeos branch is a stable base to work on, but patches need to land on
the main branch. For uploading to that directory, it's always necessary
to rebase onto that branch.

BUG=b:227535695
TEST=rebase_for_review && repo upload --cbr .

Change-Id: I5f41e52f91adf15ade84ae09f6b34dcdbe733d23
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3565627
Commit-Queue: Junichi Uekawa <uekawa@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Dennis Kempin <denniskempin@google.com>

docs/book/src/integration/chromeos.md

@@ -29,13 +29,16 @@ However, changes are not acceped to the `cros/chromeos` branch, and should be su
 `cros/main` instead.
 
 Use `repo upload -D main` to upload changes to the main branch, which works fine in most cases where
-gerrit can rebase the commit cleanly. If not, please rebase to `cros/main` manually:
+gerrit and kokoro can rebase the commit cleanly. If not, please rebase to `cros/main` manually:
 
 ```bash
 git branch --set-upstream-to cros/main
 git rebase
 ```
 
+If this doesn't work, use `tools/chromeos/rebase_for_review` to rebase the changes onto main and
+`repo upload` to upload changes to the main branch.
+
 ## First time setup / running tools
 
 - All CrosVM `tools` are not expected to work inside the ChromeOS chroot. As such, they must be run

tools/chromeos/rebase_for_review

@@ -0,0 +1,21 @@
+#!/bin/bash
+# Copyright 2022 The ChromiumOS Authors.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+set -e
+
+function main() {
+  local review_branch="tmp-$(date +%Y%m%d)-$RANDOM"
+  git checkout -b "${review_branch}"
+  git branch --set-upstream-to cros/main
+  git fetch cros main
+  git rebase --onto cros/main cros/chromeos "${review_branch}" || true
+  echo "
+
+Review branch ${review_branch} created!
+
+Resolve any conflicts and do review with repo upload.
+git checkout - to go back to the previous branch."
+}
+
+main