cl: add a verify flag to control git push --(no-)verify
cl: add a verify flag to control git push --(no-)verify

This adds a flag to 'jiri cl mail' that controls whether --no-verify
is sent to git push, which controls whether pre-push hooks are run.
The flag naming here is the same as 'git push' which maybe isn't the
most sensible but is at least consistent with using gerrit directly

MultiPart: 2/2
Change-Id: I95a9462c02e6e6b413d1f753d45056b4314234d6

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools