feat: add neogit support (#117)

## TODO

1. docs (vimdoc + readme) - this is a non-trivial feature
2. push luarocks version

## Problem

diffs.nvim only activates on `fugitive`, `git`, and `gitcommit`
filetypes.
Neogit uses its own custom filetypes (`NeogitStatus`,
`NeogitCommitView`,
`NeogitDiffView`) and doesn't set `b:git_dir`, so the plugin never
attaches
and repo root resolution fails for filetype detection within diff hunks.

## Solution

Two changes:

1. **`lua/diffs/init.lua`** — Add the three Neogit filetypes to the
default
`filetypes` list. The `FileType` autocmd in `plugin/diffs.lua` already
handles them correctly since the `is_fugitive_buffer` guard only applies
   to the `git` filetype.

2. **`lua/diffs/parser.lua`** — Add a CWD-based fallback in
`get_repo_root()`.
After the existing `b:diffs_repo_root` and `b:git_dir` checks, fall back
to
`vim.fn.getcwd()` via `git.get_repo_root()` (already cached). Without
this,
   the parser can't resolve filetypes for files in Neogit buffers.

Neogit's expanded diffs use standard unified diff format, so the parser
handles
them without modification.

Closes #110.

lua/diffs/highlight.lua

@@ -443,18 +443,11 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
     end
 
     if opts.highlights.background and is_diff_line then
-      pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, 0, {
-        end_row = buf_line + 1,
-        hl_group = line_hl,
-        hl_eol = true,
-        priority = p.line_bg,
-      })
+      local ext = { line_hl_group = line_hl, priority = p.line_bg }
       if opts.highlights.gutter then
-        pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, 0, {
-          number_hl_group = number_hl,
-          priority = p.line_bg,
-        })
+        ext.number_hl_group = number_hl
       end
+      pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, 0, ext)
     end
 
     if is_marker and line_len > pw then