This PR adds the `git.hunk_style` setting, allowing setting an alternate
style for hunks – specifically the rendering of unstaged hunks.
It has 2 options:
- `transparent` (unstaged hunks are more transparent/less opaque than
staged hunks)
- `pattern (unstaged hunks are indicated by a visual pattern)
We'll possibly explore a VSCode-style "don't show staged hunks", but the
complexity it adds is a bit out of scope for now.
Transparent:

Pattern:

Release Notes:
- Git Beta: Added `git.hunk_style` setting to allow toggling between git
hunk visual styles.