On the 'issue-3348-dev' branch: Teach status, diff, and the
lower-level working copy APIs to ignore directories when filtering by
changelist, even when using the magical empty-string changelist name.

Among other niceties, this makes `svn up --cl ""` only update
individual files which aren't in a changelist, rather than effectively
doing the likes of this:

   svn up --depth infinity .
   svn up --depth infinity ./A
   svn up --depth infinity ./A/B
   svn up --depth infinity ./A/B/E
   ...

* subversion/libsvn_client/status.c
  (tweak_status): When filtering on changelists, drop all directories,
    period.

* subversion/libsvn_wc/diff_local.c
  (diff_status_callback): Same here.

* subversion/libsvn_wc/adm_ops.c
  (svn_wc__internal_changelist_match): Tweak this filtering function
    to drop all directories, which have no business in
    changelist-related discussions.

* subversion/tests/cmdline/changelist_tests.py
  (empty_pseudo_changelist): Tweak test behavior and expectations.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/issue-3348-dev@1428602 13f79535-47bb-0310-9956-ffa450edef68