In the merge code, where doc strings require that an RA session is parented at a particular URL, assert this.  Adjust symmetric merge code to obey this.
In the merge code, where doc strings require that an RA session is parented
at a particular URL, assert this.  Adjust symmetric merge code to obey this.

* subversion/libsvn_client/merge.c
  (session_url_is, session_url_encompasses): New functions, only compiled in
    SVN_DEBUG builds.
  (fix_deleted_subtree_ranges, remove_noop_subtree_ranges,
   merge_cousins_and_supplement_mergeinfo, find_unmerged_mergeinfo,
   calculate_left_hand_side, find_reintegrate_merge): Assert that RA
    sessions are as documented.
  (do_symmetric_merge_locked): Create RA sessions parented at the right URLs.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1366120 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion