Sometimes CL author is also listed as OWNERS but want to find a co-owner
to review CL. Currently "git cl owners" is unhelpful as it consider the
author's ownership as enough.

Add a --ignore-self parameter to "git cl owners" that will cause the
tool to ignore author's ownership when looking for a reviewer.

Bug: none
Change-Id: Iba110a465a552cd6befb46c77b2e65f60b663a13
Reviewed-on: https://chromium-review.googlesource.com/c/1459625Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>