Speed up 'svn log -g' to almost the same speed as 'svn log'. Also, providea basis for speeding up other mergeinfo detection based code.The problem is that svn_fs_path_change2_t used to report mergeinfo changesas simple prop changes. Since most files are being created with defaultprops attached to them, this creates a lot of unnecessary property readand comparison operations.The idea is to a derive a mergeinfo-mod flag from the FS propset functionand store that in FSFS f7+ repositories (FSX will follow later). An extendedsvn_fs_path_change2_t makes the information visible to the user.* subversion/include/svn_fs.h (svn_fs_path_change2_t): Add mergeinfo-mod as a tristate since we don't have that information in old repositories.* subversion/libsvn_fs_util/fs-util.c (svn_fs__path_change_create_internal): Make sure the new member is always initialized properly.* subversion/libsvn_repos/log.c (fs_mergeinfo_changed): Use the new information to skip unnecessary node prop processing.* subversion/libsvn_fs_fs/structure (): Extend documentation to cover the new flag as well.* subversion/libsvn_fs_fs/fs.h (SVN_FS_FS__MIN_MERGEINFO_IN_CHANGES_FORMAT): New format capability flag.* subversion/libsvn_fs_fs/low_level.c (read_change): Detect the presense of the mergeinfo-mod flag and parse it if found. (write_change_entry): Write the mergeinfo-mod flag dependent on whether its value is known and the new INCLUDE_MERGEINFO_MODS parameter has been set. (svn_fs_fs__write_changes): Include mergeinfo-mod only if supported by the current FS format.* subversion/libsvn_fs_fs/transaction.h (svn_fs_fs__add_change): Add mergeinfo-mod parameter.* subversion/libsvn_fs_fs/transaction.c (replace_change): Copy mergeinfo-mod info as well. (fold_change): Combine mergeinfo-mod info as well. (svn_fs_fs__add_change): Store mergeinfo-mod info as well.* subversion/libsvn_fs_fs/tree.c (add_change): Add mergeinfo-mod pass-through parameter. (fs_change_node_prop): Determine whether merge info was touched. (fs_make_dir, fs_delete_node, copy_helper, fs_make_file, apply_textdelta, apply_text): These never modify mergeinfo.