auto-stack logic selects wrong repo format

If your 'branch' format does not support stacking, we select a new format that does. However, this ignores the existing repository format (which may already support stacking).

Consider this case:

$ bzr init-repo --1.9 --tree source
Shared repository with trees (format: 1.14 or 1.9)
Location:
  shared repository: source

$ bzr init --pack-0.92 source/old_branch
Created a repository tree (format: unnamed)
Using shared repository: C:/Users/jameinel/dev/,tmp/source/

$ bzr commit --unchanged -m 'first rev' source/old_branch
Committing to: C:/Users/jameinel/dev/,tmp/source/old_branch/
Committed revision 1.

$ bzr branch --stacked source/old_branch new_branch
Source format does not support stacking, using format: '1.6'
  Packs 5 (adds stacking support, requires bzr 1.6)

Created new stacked branch referring to file:///.../source/old_branch/.

$ bzr info new_branch/
Standalone tree (format: 1.6)
Location:
  branch root: new_branch

Related branches:
  parent branch: source/old_branch
     stacked on: source/old_branch

So it upgraded the branch format from 'pack-0.92' (Branch6) to '1.6' (Branch7), but it *downgraded* the repository format from '1.9' to '1.6'.

This is confusing, although not broken, for current formats. I encountered it when implementing stacking for dev6 formats. Where it turns out to downgrade from --development6-rich-root to --1.6-rich-root, which goes across a serialization format boundary, and thus falls over.