Code review comment for lp:~spiv/bzr/stacking-friendly-revision-history-verb

This fixes bug 380314. It is a cheap fix that simply falls back to VFS for
RemoteBranch._gen_revision_history if the branch is stacked. I've added tests
for both the blackbox case of "'bzr pull -r 123' from stacked" and a per-branch
test for .revision_history when there is stacking.

I have a followup patch in the works that defines a new RPC that avoids VFS
entirely in this case, but it's close to the release and getting the correctness
fix merged is more urgent.