When merging a file, don't use the pristine install preparation code to
create a tempfile, but retrieve the right location properly.

* subversion/libsvn_wc/merge.c
  (includes): Stop including adm_files.h. Use other includes directly.
  (merge_file_trivial): Create temp file in tempdir, instead of abusing
    pristine magic.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1577066 13f79535-47bb-0310-9956-ffa450edef68