When no -v flag is given, the verbosity defaults to None instead of 0,
so set 0 as a default.

Bug: 1058315
Change-Id: Id69e62550b33cd18bb1e5aed9cdc3c94f1488516
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2088193Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>