Puller/scanner raises UnstackableRepositoryFormat for some branches

[Jonathan Lange]

We are getting many OOPSes like this:

1286 UnstackableRepositoryFormat: The repository 'lp-mirrored:///~jelmer/gtk/trunk/.bzr/repository'(<RepositoryFormatKnitPack4>) is not a stackable format. You will need to upgrade the repository to permit branch stacking.
   Other: 1286 Robots: 0 Local: 0
   1286 https://code.launchpad.net/~jelmer/gtk/trunk (Unknown)
       OOPS-988SMS1004, OOPS-988SMS1010, OOPS-988SMS1016, OOPS-988SMS1022, OOPS-988SMS1028

This occurs in the scanner in released Launchpad and in the puller in trunk. It's caused by the failure to catch UnstackableRepositoryFormat when getting stacked-on URLs.

AFAICT, the bug affects branches that have a branch format that supports stacking and a repository format that doesn't. This is likely to be triggered by users running 'bzr upgrade' on branches in shared repositories without first upgrading the repository.

[Jonathan Lange (jml)]

RF 7019