pipeline doesn't fully support native colocation

[Aaron Bentley]

With bzr 2.5, bzr supports colocated branches natively. Pipeline should not impose a separate form of colocation unless specifically requested to. It seems difficult to auto-detect whether to create new pipes using sibling-style or native colocation. When adding the second pipe, it may make sense to use sibling-style, but it could also make sense to use colocation. As a transition, perhaps it would make sense to use native colocation only when already active, and to recommend switch -b in place of reconfigure-pipeline.