Code review comment for lp:~spiv/loggerhead/shared-repo-fix

This fix, combined with <https://code.launchpad.net/~spiv/bzr/bzr-http-jailbreak-bug-348308/+merge/13698>, fixes HPSS access for branches in a shared repo. I tested this via "serve-branches path/to/a/shared-repo" and then doing "bzr revno http://localhost:8080/branch-in-that-repo".

Hopefully the code is reasonably self-explanatory. Failing that it's at least simple ;)