Revision 1494913

Fix a severe performance regression in 'svn log':  The new log code
will run svn_client__repos_location_segments even if we log on the
root folder.  That triggered a long-standing performance issue in
fs_node_origin_rev.  This patch fixes the latter.

* subversion/libsvn_fs_fs/tree.c
  (fs_node_origin_rev): special case root node IDs instead of walking