Currently bot_update and tryserver crashes at initialization step if
the input contains multiple gerrit changes. This is not friendly for
CrOS CLs, e.g. crrev.com/c/2536761.

This CL moves the “crash” out of initialization, thus the consumer
of these modules could control which gerrit change to pass to bot_update
and tryserver.

This CL does NOT enable the modules here to support multiple patchset,
but allows consumers to choose one.

Bug:1146487
Change-Id: Iec46a8041189912e34da9bc5e8ff4611123ec9e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2528670
Commit-Queue: Xinan Lin <linxinan@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>