Enhance rbt post to automatically determine the existing review request
rbt -u automatically determines the existing review request to update. It 

suggests a pending review request by the current user in the current

repository, based on the request summary and description. 
If the summary and description exactly match those of an existing request, 

the request is immediately updated. Otherwise, the user is prompted to confirm

a suggestion from a ranked list of potential matches.
Reviewed at https://reviews.reviewboard.org/r/4699

-Tested with Git, Mercurial, and Bazaar using, with and without 'revision range'.

-Unit tests pass.