If a call to svn_fs_mergeinfo__update_index is ultimately going to be a no-op, don't perform any write operations at all.
The upshot of this change is that commits that don't change any svn:mergeinfo will no longer block (or be blocked by) commands that read from the merge tracking database.
* subversion/libsvn_fs_util/mergeinfo-sqlite-index.c (table_has_any_rows_with_rev): New helper function. (clean_tables): New helper function to do the cleanup of failed transactions, factored out of svn_fs_mergeinfo__update_index. (svn_fs_mergeinfo__update_index): Factor out clean_tables.
* subversion/libsvn_fs_util/sqlite-util.c * subversion/libsvn_fs_util/sqlite-util.h (svn_fs__sqlite_step): New helper for calling sqlite3_step when you don't know if you expect SQLITE_ROW or SQLITE_DONE.