Make the few remaining places that prepare files for installing use
svn_wc__db_pristine_prepare_install() to allow further refactoring the
pristine install work.

* subversion/libsvn_wc/merge.c
  (merge_file_trivial): Use the same api to obtain a tempdir as the rest
    of the merge code.

* subversion/libsvn_wc/wc_db.h
  (svn_wc__db_pristine_get_tempdir): Remove function.

* subversion/libsvn_wc/wc_db_pristine.c
  (svn_wc__db_pristine_get_tempdir): Remove function.
  (pristine_get_tempdir): Integrate svn_wc__db_pristine_get_tempdir.

* subversion/tests/libsvn_wc/pristine-store-test.c
  (write_and_checksum_temp_file): Remove function.
  (pristine_write_read,
   pristine_delete_while_open,
   reject_mismatching_text): Use svn_wc__db_pristine_prepare_install.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1577215 13f79535-47bb-0310-9956-ffa450edef68