re-added support for ghosts in newformat

[Martin Pool]

Hi,

The upgrade command in the newformat tree now carries across
references to 'ghost' revision, which are named in a revision
reference but whose data is not present in the tree.  The main use
case for these is importing data from bazaar and arch, where trees
commonly have such things.  They might also be useful in letting
people store only a subset of history locally.

There is also a test for this, and for upgrade in general, and some
new test helpers for building trees with given contents.

The test (as they tend to do) discovered a regression in the upgrade
code, which is (I think) that merging in Transport broke support for
upgrading from branches whose stores contain a mix of compressed and
uncompressed files.  That's perhaps a wierd configuration but it does
happen.  So we should decide whether to make everything compressed
before upgrading, or perhaps undo the split into CompressedTextStore
and TextStore.

--
Martin