switch -b silently ignores revision spec

When creating a new branch with switch, -r should either be an error, or it should indicate the revno in the current branch to base the new branch on. Instead, it creates a new branch at the last_revision of the current branch, but changes the working tree to the specified revision:

$ bzr switch -b -r 15666 restore-queue-test
Updated to revision 15675.
Switched to branch: /home/abentley/launchpad/colo/.bzr/branches/restore-queue-test/
$ bzr revno
15675

However, the contents match -r 15666:
$ bzr diff -r 15666
$