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

> > This is false. If the branches are not related, merging a subdir can avoid
> very
> > many unnecessary conflicts while still generating a single tree.
>
> If the branches are not related there is no way in builder to merge
> them at this point anyway (no way to specify a merge base). I haven't
> tested to make sure that the API works the same as the UI here, so
> apologies if I mispoke.

You're right about that. It would be easy to catch UnrelatedBranches and if a subpath is specified try again with a base of NULL_REVISION. Or we could add a 'mergeunrelated' directive. Or further extend the syntax to allow specifying bases.

Do you have a preference for one of those options, or do you feel this is the wrong track entirely? It seems to me that however it is specified a partial merge is a fairly neat way to provide "please give me this branch, plus the debian/ dir from another branch."

I'm sure there are many cases where blindly combining a debian/ dir from an existing package with an arbitrary version of upstream won't work. I think it's likely it will
work ok for many nightly packages though: have a recipe that specifies "current trunk + packaging from $recent-packaging". I'm not certain which use-case Robert had in mind, but that's what I'm thinking of.