Add `git.hunk_style` setting for gutter hollow hunk behavior (#26816)

This is a follow up to #26809, introducing `git.hunk_style` setting to
control whether staged or unstaged hunks are shown as hollow.

Reused `GitHunkStyleSetting` which was left over from #26504.

Release Notes:

- Added `git.hunk_style` setting to control whether staged or unstaged
hunks are hollow.