Code review comment for lp:~spiv/bzr-builder/merge-subdirs-479705

So, in two parts:
 - Andrew is looking at making sure it works with unrelated branches.

Use cases: as we discussed, for launchpads 'build from branch' to be successful for upstreams, they need easy access to debian/ in package branches that are likely to have no history relationship: This syntax allows them to specify that that is what they want without us resorting to guessing.