Overall this seems a good way to save time - I think most of the time I'm
not looking at the other branches unless I'm merging a backport and then I
generally have a second checkout I use for that.  We'd want an easy way to
still get those branches though.

Would I still be able to run 'git branch -r' to see the branches?  And just
take a hit the first time I pulled one?  Or would I not even know they
existed and have to do some tricks to pull them?

Would it be possible to add a way to annotate the manifest with "also sync
these branches" or to modify something in the git metadata to do the
equivalent?  I feel there are times (updating to new kernel versions) where
people working in the tree would care about multiple branches at once.
 It'd be nice to make things simpler in those cases.