Adding svn_wc__db_status_omitted

The new SVN_EXPERIMENTAL_COPY code does a couple of new things:

- when copying a directory it uses the "insert a complete directory
  with incomplete children" approach.

- when copying an excluded node (as the child of a copied parent) it
  puts the excluded node in the WORKING_NODE table rather than the
  BASE_NODE table.

At present I'm using svn_wc__db_status_incomplete and
svn_wc__db_status_excluded for these sorts of working node, but this
causes problems. The main one is that when svn_wc__db_read_info
returns one of these as a status value the caller cannot easily
determine if it represents a base node or a working node.

We could fix this by adding new status values (I was thinking of using
svn_wc__db_status_omitted for copies of svn_wc__db_status_excluded)
but an alternative would be to change/extend the base_shadowed flag to
something that allowed the caller to determine the state.

Anyone have any opinions?

-- 
Philip