Prepare the 'svnfsfs load-index' code for being split up into UI,
FSFS internal logic and a private interface in between.

This patch introduces the interface structure but does not move
any code around.  We only have to factor the actual FS access
into a separate function.

* subversion/svnfsfs/load-index-cmd.c
  (svn_fs_fs__load_index): New function, code taken from load_index().
  (load_index): Call the new function.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632863 13f79535-47bb-0310-9956-ffa450edef68