This wasn't part of the explicit spec for #1095, but it makes logical sense and is how the original MarkdownEditing plugin works.

Currently when indenting a bullet in markdown mode (i.e. with tab), it calls the cycleBullet function to switch which bullet is being used at the given depth (in order * > - > + > *). Ideally, when we de-indent (i.e. with shift+tab) it would do the same in reverse and cycle through bullets in reverse (in order * > + > - > *). This way, indenting and deindenting a bullet maintains the same bullet character used at the same depth.