To speed up svn_fs_node_origin_rev() (which is used exclusively by svn_repos_node_location_segments(), which is used *all over* the merge tracking code), it would be handy to have an index in the filesystem backends which maps a line of history (probably identified by a NODE_ID) to the revision in which that line of history came into being. I've done this already for the BDB backend, mapping NODE_ID to the first NODE_REVISION_ID in that line of history, from whose TXN_ID component we can get the origin revision. (I chose to map to NODE_REVISION_ID, though, because if we later have node-successor relationships, I have a great starting point for forward history crawling.) But we need something similar for FSFS. And *ideally*, we'd provide tools for populating that index for existing repositories recently upgraded to 1.5.