Sync'ing FSX with FSFS:
Merge r1554711 from /subversion/libsvn_fs_fs into subversion/libsvn_fs_x.
Conflicts were due to name prefix differences (svn_fs_x vs svn_fs_fs).

This ports the "directories are arrays now" change.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1595277 13f79535-47bb-0310-9956-ffa450edef68