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

[Hi-Angel]

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: #810

[condy0919]

@jojojames What's your thoughts?

[jojojames]

Wouldn't you want to move the key somewhere else than to remove it entirely?

[Hi-Angel]

Sorry, I didn't know what modes motion includes. Is it just normal and visual? If that's the case, then indeed your idea is a good one, moreover, I think it's better to move every single-letter command that does not respect selection to under the "normal".

[Hi-Angel]

It's just that I don't know, the motion state does not seem to be documented. There's this answer which implies it is completely unrelated to "normal" and "visual" and instead it is for read-only buffers like *Help*. But that doesn't make sense to me because from the bug we can clearly see that it influences visual state in an editable buffer/major-mode.

[jojojames]

motion -> normal is fine.

[Hi-Angel]

@jojojames done, please see if it looks okay now

[Hi-Angel]

Actually, wait, I see another command that may be confusing when used in "visual"

[Hi-Angel]

@jojojames okay, so, I'm afraid your idea with moving stuff under the "normal" is not working due to a bug somewhere. When I'm testing the current changes, while pressing d in "visual" I still see the hunk getting deleted even though it is no longer under "motion".

[jojojames]

Visual state is different from normal state. The changes here seem ok for fixing normal state&motion state for now. If you want to later fix visual state, we can figure that out separately.

Closing this PR, but feel free to reopen if you think this is in a good state to merge.