SVN_WC__OP_DEPTH format bump soon

The current wcng code, format 22, puts all working nodes at op-depth=2.
The full op-depth code is enabled by defining SVN_WC__OP_DEPTH and,
although no schema changes are involved, the way the database is used
makes in incompatible with format 22 clients if working nodes are
present. So SVN_WC__OP_DEPTH will involve a format bump to 23.

My current plan is to upgrade working copies automatically only if they
have no working nodes, i.e. no tree changes via add, delete, copy, etc.
Text and property modifications do not count as tree changes. Working
copies that have tree changes will need to be upgraded by running
diff/revert with a format 22 client followed by patch with a format 23
client.

Automatically upgrading with working nodes would involve rewriting the
op-depth and presence of the working nodes, as well as inserting some
additional working nodes. While it would be possible to write this
code, it's lot of work for a small return. It's only going to affect a
few developers. The 22-to-23 upgrade is distinct from the 1.6-to-wcng
upgrade. The 1.6 upgrade is the one that is important for general
users. It will, eventually, support upgrading with 1.6 tree changes.

The SVN_WC__OP_DEPTH code now passes all the regression tests apart from
db-test 7 and upgrade_tests 15, both of which expect the pre-op-depth
behaviour. My current plan is that format bump will occur towards the
end of this week.

-- 
Philip