repo/init: add --verbose flags

We don't use these for much yet, but init passes it down to the
project sync layers which already has support for verbose mode.

Change-Id: I651794f1b300be1688eeccf3941ba92c776812b5
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256454
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Jonathan Nieder <jrn@google.com>