Shows merge proposal URL when deleting a branch

[Colin Watson]

When I delete a branch from a remote on Launchpad, I'm given a merge proposal URL for the branch I'm deleting:

$ git push cjwatson :sync-signingkeys-commit-test
remote:
remote: Create a merge proposal for 'sync-signingkeys-commit-test' on Launchpad by visiting:
remote: https://code.launchpad.net/~cjwatson/launchpad/+git/launchpad/+ref/sync-signingkeys-commit-test/+register-merge
remote:
To git+ssh://git.launchpad.net/~cjwatson/launchpad
 - [deleted] sync-signingkeys-commit-test

This doesn't seem right. We should at least not show this in the case of deleting a branch; and I think there might be an argument for also not doing so when updating a branch that you've already pushed, although that might be more debatable.