[MERGE][bug #300177] Pass compression_parent as part of 'get_record_stream'

[John Arbash Meinel]

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Robert Collins wrote:
> I may have missed something here John, whats the intent of this?
> 
> [background - I considered exposing this initially, but decided it made
> more sense as a private member of factories which use compression
> parents, because its not something that makes sense for all compression
> types]
> 
> -Rob
> 

So... insert_record_stream() already asks about compression parents. It
does that in places like "do I have the compression parent so I know I'm
safe to insert this now". It seemed better to be explicit about it,
rather than assume that parents[0] is the compression parent.

Further, when vila started his work or bug 277537, he initially thought
that "bzr reconcile" would swap the parent order without regenerating
the text delta, thus having the compression parent be a non-lhs parent.
Which would break in very bad ways long-term. (If you have a stacked
branch, and you created a new full-history copy from it, any deltas that
pointed to non-lhs parents would silently be changed to point to lhs
parents, and thus end up with knit corruption.)

I figured it would be better to start making more code aware of non-lhs
compression parents early rather than late, even if we should *not* try
to do this with existing repository formats, because older clients are
"known broken" under those conditions.

John
=:->

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