Add key bindings to scroll the "Staged changes" pane

[xuyangyuio]

On the Files pane, when a partially staged file is focused, the keys for ScrollUpMain and ScrollDownMain can scroll the "Unstaged changes", but there is no key binding to scroll the "Staged changes" pane (mouse works though).

[stefanhaller]

Mouse wheel is perfect for me for this (two fingers on the track pad, actually). I wouldn't want to remember another pair of key bindings for this, especially when it can only scroll line by line, which is so slow.

But the situation is going to improve with #4429, which lets you focus the unstaged changes pane so that you can navigate it normally using arrow up/down, , and . and all the other navigation keys; and then you can press tab to switch over to staged changes and do the same there. I encourage you to try out the PR and give feedback. (It's not quite finished yet, but it should work well enough for testing already.)

[stefanhaller]

@xuyangyuio I just pushed an update to #4429, I consider this ready now.

[xuyangyuio]

@stefanhaller amazing job! close issue.