[MERGE] Teach GenericRepoFetcher to handle revisions/sigs properly

[John Arbash Meinel]

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The GenericRepoFetcher already is using data streams to logically handle
inventories and file texts. However, it was currently requesting
signatures and revisions 1-at-a-time.

The only reason I can think of for it to do so is because it doesn't
want to insert a signature that doesn't have an associated revision,
though why not just insert the revisions first then...

Anyway, this makes a rather large difference on the time to "bzr branch
bzrtools gc-repo/bzrtools". Like 20s => 8s.

I imagine it might also help a lot with some of our Knit format
regressions, as we can stream the data out of the knit repository.
(Though we are already warning there.)

There is the possibility that ^C while fetching revisions will cause you
to not fetch the associated signature.

However, we've also run into problems that ^C after fetching the
signature will cause a future fetch to fail. Because it doesn't think it
has the signature, goes to insert it...

Anyway, this is enough of a performance win for inter-repository
fetching, that I think it is worthwhile.

John
=:->
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