Disable hunk diff arrow buttons when there's only one hunk

[danilo-leal]

Closes #20817

One hunk	Multiple hunks

Release Notes:

• Fixed showing prev/next hunk navigation buttons when there is only one hunk

[Angelk90]

@danilo-leal : order of the next and preview arrows, wouldn't it make more sense the other way around?

Webstorm:

[danilo-leal]

@Angelk90 I guess so, yeah. It also makes sense with spoken language, as we always say "up and down" instead of "down and up". But I figure the current order is there given that, if you start traversing the file from its very first hunk, your next stop (first button) will be going down, and not up (unless you start checking diffs from bottom-up, and not top-down, which feels unusual). So, I'm not sure. Might hold off on this for now.