vim: Fix key navigation on folded buffer headers

[ConradIrwin]

Closes #24243

Release Notes:

• vim: Fix j/k on folded multibuffer headers

[baldwindavid]

Oh, possible this mitigates one of the related issues I mention in #25951

[ConradIrwin]

Great, thanks for that issue (and sorry for not fixing the scroll thing when you brought it up on discord weeks ago :D).

Looks like za and friends also don't work inside deleted content, I'll fix that.

Also will look into the scroll position stuff; I think we should probably use the location of your cursor as well as / instead of the scroll anchor for figuring that out.

[baldwindavid]

Great, thanks for that issue (and sorry for not fixing the scroll thing when you brought it up on discord weeks ago :D).

Eh, I've seen the git channel notes... there's a flood of things y'all are working through and this is a minor thing. I'm very impressed with the git panel already; way more than expected in phase one. Amazing work!

Looks like za and friends also don't work inside deleted content, I'll fix that.

Yeah, expand/collapse is something I know is there, but has always been finicky when trying to use it in vim so I've mostly ignored it. Multibuffers have always been cool, but take on extra shine to me now with diffs, so will probably try to leverage them more.

[ConradIrwin]

/cherry-pick v0.177.x