This makes bot_update query Gerrit when syncing for Gerrit tryjobs.

The query will establish the actual destination branch of the CL,
which can be different from master (e.g. a feature branch).

Bot_update will ensure to use this destination branch for the repo
that corresponds to the CL's project. Both the main project or a
deps'ed project work.

Initially, this lives behind a flag that can be controlled in
downstream recipes. Eventually we'll set this to default after a
gradual roll-out.

Branches in branch-heads are not supported yet.

Bug: 740456
Change-Id: I4a0d50e2ca8fe90f8d29964a3ffab17291f7be60
Reviewed-on: https://chromium-review.googlesource.com/566824
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>