Quality of life fix for rbt post when updating request

Review Request #9442 — Created Dec. 15, 2017 and submitted — Latest diff uploaded




The issue: you post a review request with a different set of target
people or groups than are in your .reviewboardrc. When you then update
this existing review request, if you don't re-specify your custom
people/groups, then rbt will overwrite your custom choices with the ones
specified in your .reviewboardrc.

With this change, rbt post will use the options in the .reviewboardrc
only when creating a new review request. If you are updating an existing
request, then it will ignore the settings in the .reviewboardrc. You can
still manually change the groups/people in your update if you would
like. Unit tests show this functionality.

Been using this at work successfully.
Manually walked through all the posting options.
Unit tests should cover each case.