Eliminate duplicate repository creation code from FSFS hotcopy.

Factor out most of svn_fs_fs__create into a svn_fs_fs__create_file_tree
function that creates all of the required files and sub-folders.  Use it
in hotcopy to create the empty target repository.

* subversion/libsvn_fs_fs/fs_fs.h
  (svn_fs_fs__create_file_tree): Declare new private API function.

* subversion/libsvn_fs_fs/fs_fs.c
  (svn_fs_fs__create_file_tree): Implement it with code taken from ...
  (svn_fs_fs__create): ... this.  Keep only the parameter checking and
                       the final 'format' file creation.

* subversion/libsvn_fs_fs/hotcopy.c
  (hotcopy_create_empty_dest): Use the new private API to create the
                               target repo and tweak it to our needs.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1629865 13f79535-47bb-0310-9956-ffa450edef68