[RFC] Smoother upgrades

[Ian Clatworthy]

I'm planning to make some improvements to the upgrade command soon,
given we'll be encourage everyone to upgrade all of their branches
shortly after 2.0 is released. If you have any ideas on what you'd like
to see, please let me know.

The sort of changes I'm thinking of are:

1. If run on a shared repo, upgrade all the branches as well

2. If run with a --find (or --recurse) option, search in subdirectories
   for repositories and branches that need to be upgraded.

3. Something intelligent wrt doing non-stacked branches before stacked
   ones - whatever makes sense here.

Thoughts?

Ian C.