Validate target working copy for mixed revisions, local modifications and 
switched subtrees before contacting server. Mixed revision working copy is 
very often case and it's better to notify user earlier before spending 
significant amount of time for merge info retrieval from server.

* subversion/libsvn_client/merge.c
  (client_find_automatic_merge): Check target WC for mixed revisions, 
   local mods and switched substrees.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1488183 13f79535-47bb-0310-9956-ffa450edef68