Make 'svn merge' honor the 'config:miscellany:preserved-conflict-file-exts'
configuration setting.

* subversion/libsvn_client/merge.c
  (merge_cmd_baton_t): Store extensions to be preserved.
  (merge_file_changed): Preserve extensions when producing labels, just like
    how our update editor did this since before 1.0.
  (do_merge): Parse and store config option.

* subversion/tests/cmdline/merge_tests.py
  (conflict_naming): New test.
  (test_list): Add conflict_naming.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1577151 13f79535-47bb-0310-9956-ffa450edef68