Fix two bugs in the BzrPoller:

Fix two bugs in the BzrPoller:

 - When re-loading a configuration file where an extra BzrPoller is added,
   there was a race condition that would cause the poller to run before
   initialisation was done, causing an exception and failing to add the
   new poller.

 - When specifying an explicit branch name for the BzrPoller, it used
   the wrong branch name when looking at old revisions during restart,
   causing it to think that the tip of every branch was new and consequently
   re-build every branch at every restart (the code for when branch_name is
   specified as SHORT also looks wrong, but is not fixed in this commit. I
   don't know if everyone uses SHORT anywat).