[fastimport/MERGE] Train fixes

[Ian Clatworthy]

John Arbash Meinel wrote:

> I'm pretty sure that fast-import doesn't handle ghosts. 'git' has an
> explicit
> property to chain their hashes back through all of history. (So if you
> changed
> the very first commit somehow all of your sha1-revision-ids would change.)
> 
> So I'm guessing that fast-export | fast-import would lose information
> about ghosts.

That's my understanding as well. You could specify parents that don't
exist but bzr fast-import and git-fast-import would currently consider
that an error.

I'm of the opinion that they ought to continue to do so, at least by
default. When generating the references to ghosts, do we know they're
ghosts at that point? If so, we could extend the stream format to
explicitly mark them as such. Alternatively, we could add an option (to
fast-import) to permit ghost references. Even if not stored initially,
at least the importer wouldn't abort at that point.

Shawn,

Any opinions on this?

Ian C.