Make 'svn add --force /path/to/wc-root' work as expected.

* subversion/libsvn_client/add.c
  (svn_client_add4): Check the to-be-added path to see if it's a
    working copy root.  If so, we know it's under version control so
    --force is required to go further.  We also know that we can't get
    a WC lock on its parent, so adjust our locking parameters
    accordingly.

Index: subversion/libsvn_client/add.c
===================================================================
--- subversion/libsvn_client/add.c	(revision 1394552)
+++ subversion/libsvn_client/add.c	(working copy)
@@ -671,6 +671,7 @@
   const char *parent_abspath;
   const char *local_abspath;
   const char *existing_parent_abspath;
+  svn_boolean_t is_wc_root;
 
   if (svn_path_is_url(path))
     return svn_error_createf(SVN_ERR_ILLEGAL_TARGET, NULL,
@@ -678,6 +679,17 @@
 
   SVN_ERR(svn_dirent_get_absolute(&local_abspath, path, pool));
 
+  /* See if we're being asked to add a wc-root.  That's typically not
+     okay, unless we're in "force" mode. */
+  SVN_ERR(svn_wc_is_wc_root2(&is_wc_root, ctx->wc_ctx,
+                             local_abspath, pool));
+  if (is_wc_root && (! force))
+    {
+      return svn_error_createf(SVN_ERR_ENTRY_EXISTS, NULL,
+                               _("'%s' is already under version control"),
+                               svn_dirent_local_style(local_abspath, pool));
+    }
+  
   /* ### this is a hack.
      ### before we switched to absolute paths, if a user tried to do
      ### 'svn add .', PATH would be "" and PARENT_PATH would also be "",
@@ -708,7 +720,9 @@
     add(local_abspath, depth, force, no_ignore, existing_parent_abspath,
         ctx, pool),
     ctx->wc_ctx,
-    existing_parent_abspath ? existing_parent_abspath : parent_abspath,
+    is_wc_root ? local_abspath
+               : (existing_parent_abspath ? existing_parent_abspath 
+                                          : parent_abspath),
     FALSE /* lock_anchor */, pool);
   return SVN_NO_ERROR;
 }