Currently, for updating a review request a -r option is needed for post-
review. However, while using Git it might make sense to set that review
request ID to a "reviewboard.id.$branch" variable so that post-review can
reuse that on consequent calls on that branch.
E.g.:
$ git status
# On branch XYZ
$ git config --get reviewboard.id.XYZ
$ post-review --server http://server.local/
Review request #1234 posted.
$ git config --get reviewboard.id.XYZ
1234
$ post-review
Review request #1234 posted.