[MERGE] insert record streams with missing compression parents.

[Andrew Bennetts]

Robert Collins wrote:
> This patch allows insert_record_stream to accept a stream with deltas
> where the basis texts are not available at all without erroring
> *immediately*. Instead the error is deferred as late as possible. For
> kndx ('knit' format repositories) index objects that is the end of the
> record stream. For pack repositories it is commit_write_group(). When
> combined with the suspend/resume functionality we've been working on,
> this will allow two calls to a stateless smart server to handle pushing
> texts to a stacked repository on a smart server.

This all looks good to me.

bb:approve

-Andrew.