diff-mode: disable single-letter cmds w/o respect to selection in "visual"

[Hi-Angel]

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As mentioned in the original issue, the keybinding is confusing and problematic. There has been no opposing comments for 1.5 months, so let's go ahead and ~~remove~~ make it to only work in normal.

Fixes: https://github.com/emacs-evil/evil-collection/issues/810