[MERGE] Allow stacking for --dev6

[John Arbash Meinel]

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The attached patch implements stacking support for
- --development6-rich-root formats.

1) I chose to just allow stacking, rather than require a format bump. No
on-disk structures really changed, and it is a dev format. Older clients
will abort complaining that it isn't stackable, but that's ok. (They
won't violate any stacking invariants.)

2) GroupcompressVersionedFiles already supported stacking, as part of
the versionedfiles interface tests I had to implement it a while ago.

3) The only real changes where to update
  a) GroupCHKStreamSource.get_stream_for_missing_keys() to return a
     stream that includes all referenced CHK pages for a given
     inventory.
  b) Update ResumedPack to support CHK indexes.
  c) Add ResumedGCPack just because I needed a way to force
     _get_external_refs() to return the empty set, rather than thinking
     it new how to understand the 'value' field of indexes.

4) There were a few tests updated, for example:

   TestPackRepository.test_suspend_write_group (et al), weren't passing
   make_repository(..., format=self.get_format()), which meant they were
   all only testing the default format.

   The only complaint here, is that I only caught this by accident (the
   test wasn't failing when I new it should.)

   I think the problem is that our other per-X tests override something
   so that 'self.make_branch()' creates the format-under-test, while
   TestPackRepository requires each test do it manually.


I'm pretty confident with this patch. I'm doing manual testing now. So
far, it is looking good.

I'd like to get this into 1.15 if possible. (Since rc1 is this Friday,
that doesn't seem onerous.)

John
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