cleanup
#167 - visual mode should not add entries to the undo stack unless modifying commands have been issued

'mark_undo_groups_for_gluing' and 'maybe_mark_undo_groups_for_gluing' in
conjunction with 'glue_marked_undo_groups' seem to add entries to the undo
stack regardless of whether intervening commands were modifying or non-
modifying. To avoid undesired entries in the undo stack (that will break
the redo feature too), we need to perform some checks.
edit changelog
Added tag 0.7.3 for changeset 5907c1e03e94