merge-into-chromeos: Create chain of merge commits
merge-into-chromeos: Create chain of merge commits

We can use the gerrit api to detect previous merge commits
the bot has made, then use those as the base for the next merge.

This will create smaller merges and is easier to manage as we do not
have to abandon so many changes.

One complication is detecting possible cherry picks the oncall may
have added to the previous merge commit.

Only open merge commits are taken into account.

BUG=b:213932296
TEST=./ci/kokoro/simulate build-merge-into-chromeos.sh

Change-Id: I795109be4c45bec49068c3e292a3947505755442
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3381832
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Tested-by: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm