upload: add a --topic option for setting topic explicitly

Let people specify the exact topic when uploading CLs.  The existing
-t option only supports setting the topic to the current local branch.

Add a --topic-branch long option to the existing -t to align it a bit
better with --hashtag & --hashtag-branch.

Change-Id: I010abc4a7f3c685021cae776dd1e597c22b79135
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/431997


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>