This simplifies workflows that tend to use a single review request per branch by maintaining an association between review request ID and branch.  Ultimately i'm aiming to have 'upsert'-like behavior where i can run a single command for posting and updating review requests.  --update sort of does this, but tends to result in unnecessary delay and prompting when multiple reviews are outstanding.  I'm also not opposed to reworking the change to integrate this behavior with --update if that is acceptable.
The name --store-id can be considered a straw man, feel free to suggest alternatives!
I'm sure there's additional interplay between command line args i'm overlooking, so guidance is appreciated there.
Additionally, i made a small change to testbase.py to avoid false failures after a failed test case. (In this case the original failure was due to a horked hg installation, but the change gave targeted failures.)

$ nosetests

...................SSSSSSSSSSSSSSS...............................SSSSSSSS.................................

Ran 106 tests in 49.260s
OK (SKIP=23)

Also used a build with this change to post and update this review :-)