feat(diff): add a unified diff view

Whole-file comparisons currently need split windows. Add a unified
layout to diff and diffthis that displays deleted lines inline, while
retaining editable working files and read-only revision buffers.

Use --unified or unified=true to start in this layout, and gu in the
file panel to toggle it. Reuse existing hunk rendering, navigation and
preview code, keeping the displayed comparison separate from the staging
base.

Retain shared hidden comparison buffers across reloads and layout
changes. Preserve cursor positions and align deleted text and base line
numbers when signs and gutter widths change.

Assisted-by: Codex