This is a reland of 3806b7fb

Original change's description:
> bot_update: Fetch references to be synced.
>
> If a solution is to be synced to a reference, such as
> refs/branch-heads/#### fetch it so that it can later be
> checked out.
>
> I tried this change on a LUCI task that runs chromium_presubmit
> for crrev.com/c/1048868. The bot_update works as expected:
> https://ci.chromium.org/swarming/task/3d5b294cff467710?server=chromium-swarm.appspot.com
>
>
> Bug: 840445
>
> Change-Id: I6444c0feb679ca89b2f4065215de915494e5082a
> Recipe-Nontrivial-Roll: infra
> Recipe-Nontrivial-Roll: build
> Recipe-Nontrivial-Roll: build_limited
> Recipe-Nontrivial-Roll: build_limited_scripts_slave
> Reviewed-on: https://chromium-review.googlesource.com/1049007
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
> Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>

Bug: 840445

Recipe-Nontrivial-Roll: infra
Recipe-Nontrivial-Roll: build
Recipe-Nontrivial-Roll: build_limited_scripts_slave
Change-Id: I64a97aafe5fab9a26c478262e9ca104dc59e61f5
Reviewed-on: https://chromium-review.googlesource.com/1052387
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Michael Moss <mmoss@chromium.org>