In the spirit of r28059, make sure never to do a SELECT query that could possibly return info about an uncommitted revision. index_path_mergeinfo wants to find out about mergeinfo that is *older* than the revision being committed (new_rev), so it should pass new_rev-1 to get_mergeinfo_for_path. (This shouldn't be a behavior change, since any row with revision=new_rev was already deleted in svn_fs_mergeinfo__update_index.)
* subversion/libsvn_fs_util/mergeinfo-sqlite-index.c (index_path_mergeinfo): Query mergeinfo that is strictly older than the mergeinfo we're inserting.
* subversion/libsvn_fs_util/mergeinfo-sqlite-index.c Add a note at the top warning that all queries on the mergeinfo tables must actively ensure that they don't get rows corresponding to uncommitted revisions.