[MERGE][BUG #153786] Autopack retries

[John Arbash Meinel]

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This is another step in the "get pack actions to retry when pack files
move."

In this case, I only worked on supporting "autopack" functionality. With
the basic result that if the pack files change, we start from scratch.
For "autopack" this seems perfectly fine, because most likely this is
caused by someone else actually completing an autopack while we are
trying to do our own.

I was originally thinking to make it more generic, so that any Packer
based fetch would work. But I feel like we are moving towards the
"get_record_stream()" api anyway, which I already gave the ability to
restart.

I had been working on having Packer not start from scratch each time,
and that gets hard to figure out exactly what needs to be copied, what
has already been copied, and where the new data should be put. (Start a
new pack, try to append to the existing one, etc.)

Anyway, this is still better than what we had, because it will allow
concurrent autopacks to succeed.

John
=:->
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