lp-propose should ignore superseded proposals

[Stuart Bishop]

bzr refuses to create my merge proposal, because the current branch already has a superseded proposal

$ bzr lp-propose
bzr: ERROR: There is already a branch merge proposal: https://code.launchpad.net/~stub/launchpad/staging/+merge/70568

$ bzr version
Bazaar (bzr) 2.3.4
  Python interpreter: /usr/bin/python 2.7.1
  Python standard library: /usr/lib/python2.7
  Platform: Linux-2.6.38-10-generic-x86_64-with-Ubuntu-11.04-natty
  bzrlib: /usr/lib/python2.7/dist-packages/bzrlib
  Bazaar configuration: /home/stub/.bazaar
  Bazaar log file: /home/stub/.bzr.log

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[Martin Pool (mbp)]

I thought this was a dupe, but I can't find it. There have been some similar server-side errors in Launchpad, but now it should be possible to fix this entirely in bzr.