upload: add a --hashtag-branch option akin to -t

This will automatically add the current local branch name as a hashtag.

Bug: https://crbug.com/gerrit/10477
Change-Id: I888f8be8419c801f2d98b7a2ad2486799e94f32c
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255893
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>