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
change variable name
fix tests for marks; add more tests
edit comments
Merge branch 'master' of git://github.com/guillermooo/Vintageous