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.

spec/parser_spec.lua

@@ -587,5 +587,86 @@ describe('parser', function()
       assert.are.equal('/tmp/test-repo', hunks[1].repo_root)
       delete_buffer(bufnr)
     end)
+
+    it('detects neogit modified prefix', function()
+      local bufnr = create_buffer({
+        'modified   hello.lua',
+        '@@ -1,2 +1,3 @@',
+        ' local M = {}',
+        '+local x = 1',
+        ' return M',
+      })
+      local hunks = parser.parse_buffer(bufnr)
+
+      assert.are.equal(1, #hunks)
+      assert.are.equal('hello.lua', hunks[1].filename)
+      assert.are.equal('lua', hunks[1].ft)
+      assert.are.equal(3, #hunks[1].lines)
+      delete_buffer(bufnr)
+    end)
+
+    it('detects neogit new file prefix', function()
+      local bufnr = create_buffer({
+        'new file   hello.lua',
+        '@@ -0,0 +1,2 @@',
+        '+local M = {}',
+        '+return M',
+      })
+      local hunks = parser.parse_buffer(bufnr)
+
+      assert.are.equal(1, #hunks)
+      assert.are.equal('hello.lua', hunks[1].filename)
+      assert.are.equal('lua', hunks[1].ft)
+      assert.are.equal(2, #hunks[1].lines)
+      delete_buffer(bufnr)
+    end)
+
+    it('detects neogit deleted prefix', function()
+      local bufnr = create_buffer({
+        'deleted   hello.lua',
+        '@@ -1,2 +0,0 @@',
+        '-local M = {}',
+        '-return M',
+      })
+      local hunks = parser.parse_buffer(bufnr)
+
+      assert.are.equal(1, #hunks)
+      assert.are.equal('hello.lua', hunks[1].filename)
+      assert.are.equal('lua', hunks[1].ft)
+      assert.are.equal(2, #hunks[1].lines)
+      delete_buffer(bufnr)
+    end)
+
+    it('detects bare filename for untracked files', function()
+      local bufnr = create_buffer({
+        'newfile.rs',
+        '@@ -0,0 +1,3 @@',
+        '+fn main() {',
+        '+    println!("hello");',
+        '+}',
+      })
+      local hunks = parser.parse_buffer(bufnr)
+
+      assert.are.equal(1, #hunks)
+      assert.are.equal('newfile.rs', hunks[1].filename)
+      assert.are.equal(3, #hunks[1].lines)
+      delete_buffer(bufnr)
+    end)
+
+    it('does not match section headers as bare filenames', function()
+      local bufnr = create_buffer({
+        'Untracked files (1)',
+        'newfile.rs',
+        '@@ -0,0 +1,3 @@',
+        '+fn main() {',
+        '+    println!("hello");',
+        '+}',
+      })
+      local hunks = parser.parse_buffer(bufnr)
+
+      assert.are.equal(1, #hunks)
+      assert.are.equal('newfile.rs', hunks[1].filename)
+      delete_buffer(bufnr)
+    end)
   end)
 end)