Enable rbt stamp for Perforce
Previously, rbt stamp supported only git. I want to use it for perforce too,

so this change accomplishes that. I've tried to make stamp behave as similarly

as possible to post, with respect to the way a changelist is specified.
For perforce, rbt post is able to "update" an existing review without the -u

flag by trying to post the changelist to a new review. ReviewBoard then returns

an exception stating that the changelist already has a review, and then rbt

updates that review instead. To imitate this behavior in rbt stamp, since I

didn't want to actually have to post a review, I implemented a "guessing

strategy" that queries ReviewBoard for a review request associated with the

particular changelist ID. If one is found, that is the review whose URL is

stamped.
I would also like to add a stamp option directly into rbt post, so that a

review can be posted and stamped in one command (this would also eliminate

ambiguity about which commit the stamp is attached to), but that will come in a

a separate code review. This would also allow for an additional useful guessing

strategy, to find the review request ID by searching the commit message for a

URL.

Tested manually by posting and stamping many draft reviews with Perforce and Git.