Follow-up to r1443040, reuse a session.

* subversion/libsvn_client/merge.c
  (do_automatic_merge_locked): Pass the session we open to normalize merge
   sources to do_merge().



git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1443070 13f79535-47bb-0310-9956-ffa450edef68