git-cl: Use buildbucket v2 to fetch tryjob results.

Bug: 976104
Change-Id: Icf761f1cd093f7600ad43b71af474e52780f1997
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1842335Reviewed-by: Anthony Polito <apolito@google.com>
Reviewed-by: Andrii Shyshkalov <tandrii@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>