upload: add support for --yes

This adds a CLI option to the existing autoupload gitconfig knob that
allows people to automatically answer "yes" to the various prompts.

Bug: https://crbug.com/gerrit/12368
Change-Id: I819ebca01b9a40240b33866ae05907c7469703e3
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255892
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>