upload: add a --ignore-hooks option

When upload hooks fail, people are forced to use --no-verify to upload
CLs anyways.  When projects have flaky hooks, this trains people to
always use that option.  This is obviously bad: hooks might get fixed,
or some of the hooks are always good & people should review.

Lets add an --ignore-hooks option.  This still runs the hooks, but any
failures will be ignored and allow the user to upload anyways.

Bug: https://crbug.com/gerrit/12230
Change-Id: Ide2ac8a40a656bfcd6aae20c3ce8118e06bf909b
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254452
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>