Make git panel entries clickable (#22329)

Makes a first pass over git panel UI, making it more interactive.


![image](https://github.com/user-attachments/assets/4d43b086-4ef2-4913-9783-2b9467d99c9a)


* every item can be selected, the selection is shown in the panel
* every item can be clicked, which changes the selection and
creates/focuses the editor with a project changes multi buffer
* the editor is scrolled so that the clicked item is in the center
* it's possible to nagivate up and down the panel, selecting
next/previous items in it, triggering the editor scroll

Known issues:

* entries are updated oddly sometimes (should become better after
DiffMap improvements land?)
* only unstaged diffs are shown currently (entry status storage should
help with this)
* no deleted files are displayed (the underlying work is done by others
now)
* added files have no diff hunks shown (DiffMap will have it?)
* performance story has not improved (again, DiffMap and status storage
should help with this)

Release Notes:

- N/A