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.

README.md

@@ -2,14 +2,15 @@
 
 **Syntax highlighting for diffs in Neovim**
 
-Enhance `vim-fugitive` and Neovim's built-in diff mode with language-aware
-syntax highlighting.
+Enhance [vim-fugitive](https://github.com/tpope/vim-fugitive),
+[Neogit](https://github.com/NeogitOrg/neogit), and Neovim's built-in diff mode
+with language-aware syntax highlighting.
 
 <video src="https://github.com/user-attachments/assets/24574916-ecb2-478e-a0ea-e4cdc971e310" width="100%" controls></video>
 
 ## Features
 
-- Treesitter syntax highlighting in fugitive diffs and commit views
+- Treesitter syntax highlighting in vim-fugitive, Neogit, and `diff` filetype
 - Character-level intra-line diff highlighting (with optional
   [vscode-diff](https://github.com/esmuellert/codediff.nvim) FFI backend for
   word-level accuracy)
@@ -37,6 +38,21 @@ luarocks install diffs.nvim
 :help diffs.nvim
 ```
 
+## FAQ
+
+**Does diffs.nvim support vim-fugitive/Neogit?**
+
+Yes. Enable it in your config:
+
+```lua
+vim.g.diffs = {
+  fugitive = true,
+  neogit = true,
+}
+```
+
+See the documentation for more information.
+
 ## Known Limitations
 
 - **Incomplete syntax context**: Treesitter parses each diff hunk in isolation.