magit-visit-item: move definition
magit-visit-item: cosmetics
magit-visit-item: don't needlessly use call-interactively
collect all ediff wrappers in one place
magit-interactive-resolve: absorb magit-interactive-resolve-item
magit-ediff-buffers{,3}: new functions

Add `magit-ediff-buffers' and `magit-ediff-buffers3',
replacing `magit-ediff*'.