In any buffer you can type C-hm to see the major and minor mode descriptions, which almost always includes the key bindings defined by each mode (and if it doesn't, you can type C-hb and then search for the mode you're interested in).
Major mode magit-diff-mode
includes the following bindings:
M-1 magit-show-level-1-all
M-2 magit-show-level-2-all
M-3 magit-show-level-3-all
M-4 magit-show-level-4-all
M-H magit-show-only-files-all
M-S magit-show-level-4-all
M-g magit-goto-diffstats
M-h magit-show-only-files
M-n magit-goto-next-sibling-section
M-p magit-goto-previous-sibling-section
M-s magit-show-level-4