Before this commit, we would select a non-visible entry when a directory
is collapsed. Now we correctly select the visible entry that is visually
the previous/next entry in the list.

**Note**: I removed the `cx.notify()` call as it's already part of the
`self.scroll_to_selected_entry(cx)` call. So we don't notify twice :).

Follow-up: https://github.com/zed-industries/zed/pull/45002

**Before**


https://github.com/user-attachments/assets/da0b8084-0081-4d98-ad8a-c11c3b95a1b7

**After**


https://github.com/user-attachments/assets/8a16afb0-fdde-4317-b419-13143d5d608e

Release Notes:

- git_ui: Fix select next/previous entry selects non-visible entry when
tree view is enabled