WC format bump: store conflict files by relpath rather than name

Text and property conflict file names are stored in ACTUAL_NODE as
simple names and the WC code has to do path manipulation to obtain the
path from name. For the property conflict file the manipulation is
different for files and directories and this is a problem for the new
revert code as it may have removed the NODES row that contained the node
kind.

To fix this I plan to store relpaths instead of names. There will be no
change to the SQLite tables, just to the way the existing columns are
used by the code. I'll bump the WC format and autoupgrade existing
working copies.

-- 
Philip