Notable changes:
  * New API for luci.notifier(...).
  * proto.to_wirepb/proto.from_wirepb.
  * Improvements to `semantic-diff` subcommand.

R=tandrii@chromium.org
BUG=1030067

Change-Id: Ia5a727d94c93d06d39216e588de76671e25fc60f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1949411Reviewed-by: Andrii Shyshkalov <tandrii@google.com>
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>