[MERGE] Comments and tests for repository stacking

[Martin Pool]

I investigated <https://bugs.launchpad.net/bzr/+bug/252428>.  It looks like the
concern expressed there is not actually a real bug in the code as it stands,
but it was worth checking.  This adds two (passing) tests for handling of
repacking of stacking, and some comments to make it more clear why it is as it
is.

I'm also going to add some integration-type tests for the scenario of upgrading
a stacked-on repository.  You should then get an error trying to use the
stacked repository, but it should be possible to upgrade that too and then keep
going.
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