Make file preview scrollable

[TD-Sky]

Is your feature request related to a problem? Please describe.
I often find that I enter a file and check its diff then go back to file tree. The steps can be reduced if file preview is scrollable.

Describe the solution you'd like
I have wrote some code, but gitui doesn't capture the key events of scrolling file preview.

Describe alternatives you've considered
Not yet.

Additional context
I hope gitui could scroll the right part without entering file: