[MERGE] 1.12-preview formats

[Ian Clatworthy]

This patch introduces two new experimental formats based
on a new working-tree format - WorkingTree5. Unlike previous
working trees, this new format permits content filtering and
filtered views. Separate patches for these features will be
delivered soon.

For the sake of code clarity, I've introduced this new format
in a new file - workingtree_5.py - and the new classes
subclass their wt4 counterparts. That's consistent with
how other workingtrees have been introduced fwiw. In theory,
it ought to be ever so slightly faster to make wt5 the
complex class and for wt4 to subclass it - the approach we
take with branch & repo formats. I personally don't think
the performance gain is worth the trouble/complexity but
I'm open to reconsidering this if others think otherwise.

Ian C.
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