Tidy up some merge code for storing paths in a hash.

* subversion/libsvn_client/merge.c
  (store_path): Move here from further down the file.
  (alloc_and_store_path): New function.
  (tree_conflict, tree_conflict_on_add, prepare_merge_props_changed,
   conflict_resolver, merge_file_deleted, merge_dir_deleted): Simplify by
    using store_path() and/or alloc_and_store_path().
  (pre_merge_status_cb): Simplify, removing a redundant apr_pstrdup and
    using apr_pmemdup instead of allocate + clear + copy.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1437720 13f79535-47bb-0310-9956-ffa450edef68