This PR fixes that when you are below a header and want to select a
previous item, we would select a non-visible item because we did a -1 of
the newly selected index, which could end up inside a collapsed
directory.

**Note**: This is only an issue for the git tree view mode.

**Before** (See selection is not visible anymore after select previous,
because it's inside the collapsed directory)


https://github.com/user-attachments/assets/bd032768-b947-45f8-b5a1-6185010202cf

**After** (See selection now selects the first visible item in the list
which is the collapsed directory)


https://github.com/user-attachments/assets/47f67237-be7c-4ef5-8b99-6a3a2b016086

Before you mark this PR as ready for review, make sure that you have:
- [x] Added a solid test coverage and/or screenshots from doing manual
testing
- [x] Done a self-review taking into account security and performance
aspects
- [x] Aligned any UI changes with the [UI
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)

Release Notes:

- Git UI: Fix selecting previous item would select a non-visible item if
it's inside a collapsed directory