On the 'move-tracking-2' branch: A follow-up to r1628497: expose a needed
function.

* subversion/include/private/svn_ra_private.h,
  subversion/libsvn_ra/ra_loader.c
  (svn_ra_fetch): Expose this function privately, that was called 'fetch'.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/move-tracking-2@1628515 13f79535-47bb-0310-9956-ffa450edef68