After staging, next unstaged change is janky

The logic that chooses the next unstaged change after staging a change is kind of janky and breaks my flow. I'll call this a bug, call it what you want. I _believe_ the logic is basically nothing. It just tries to keep the vertical _visual_ position in the tree of unstaged changes. Since that is a _tree_ though, it feels weird. Everything is fine if you have a flat list of files, but I usually don't.

Here's the set up:
```
mkdir -p /tmp/treebug
cd /tmp/treebug
git init

echo hunk > f
mkdir g
echo hunk > g/g
echo hunk > g/h
echo hunk > i
echo hunk > j
```
Then enter `gitui`...

## Moving into directories

Ok, let's stage `f`.

Hm, I did not move to the next file. Instead, I moved to the directory g. So just pressing enter repeatedly will not stage file by file, but sometimes stage directories. I don't like this, but whatever. The big issue arises when I am staging from the diff view. This is my initial diff view:

Now, when I stage this hunk, my next screen is the following:

I am seeing the diff of nothing, and I can't do anything in this view. I am forced to return to the tree and navigate to the next file. So staging hunk-by-hunk by repeatedly pressing enter does not work either.

## Moving out of directories

Okay, `f` has been staged. We navigated to the next file manually:

Let's stage `g`:

Good, let's stage `h`:

We just skipped `i`. That is frustrating. Again, in the file tree view, I can correct this. In the diff view, I cannot.

## The fix

Not sure, one would have to experiment. Maybe this would already work:

- After staging a _file_, move to the next _file_.
- After staging a _directory_, keep the vertical position in the GUI.

Riproduci il comportamento in gitui usando la configurazione /tmp/treebug e le viste dell’albero dei file e delle differenze descritte sopra. Traccia come viene selezionata la modifica successiva non in staging dopo aver messo in staging un file, una directory o un hunk. Il lavoro è completato quando lo staging ripetuto avanza al file successivo e non mostra un diff vuoto né salta dei file.

Scritto dal modello di indicizzazione a partire dal testo della issue.