[MERGE][bug #153786] KnitVersionedFiles retry when appropriate

[John Arbash Meinel]

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This is probably patch 2/3 for the retry code that I'm working on.

This gets the KVF interface to retry whenever the index and pack files
get 'skewed'. (We find something in the index, but when we go to read it
in a pack file it has disappeared.)

Some of this is a bit over-broad, but I was more concerned about it
working, and trying to do less is difficult. (We *could* try to figure
out exactly which records are invalid, and then only retry those rather
than everything, but as this is only fallback code, I'm not specifically
concerned about it yet.)

The last patch in this series will be to update the Packer code (which
is what is used when fetching between pack repositories). That code is a
bit harder to fix, because you are dealing with 2 repositories. The easy
fix is to just abandon the current new pack file, and restart the whole
fetch over again. I was hoping to do better than that and only refetch
the data at the current level (so if we retry after downloading
revisions and inventories then we only restart to download the text
contents.) I'll see how hard it is, and how appropriate it is. (When
doing 'autopack' for example, you want to start all over again because
you may not need to do anything.)

John
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