Parse both old and new filenames from rename lines (R old -> new).
When diffing staged renames, use old filename as base to correctly
show content changes rather than treating the file as entirely new.
Also adds comprehensive tests for filename edge cases:
- Double extensions, hyphens, underscores, dotfiles
- Deep nested paths, complex renames
- Documents known limitation with filenames containing ' -> '
Section headers (Staged/Unstaged) now show all diffs in that section,
matching fugitive's behavior. Untracked files show as all-added diffs.
Deleted files show as all-removed diffs.
Also handles edge cases:
- Empty new/old content for deleted/new files
- Section header detection returns is_header flag
Adds du/dU keymaps to fugitive's :Git status buffer for opening unified
diffs instead of side-by-side diffs:
- du opens horizontal split (mirrors dd)
- dU opens vertical split (mirrors dv)
Parses status buffer lines to extract filename and detect section
(staged/unstaged/untracked). For staged files, diffs index vs HEAD.
For unstaged files, diffs working tree vs index.
Configurable via vim.g.diffs.fugitive.horizontal/vertical (set to
false to disable).
