fix(parser): detect filetype from existing buffer

Files detected via shebang or modeline (e.g., `build` with `#!/bin/bash`)
weren't getting syntax highlighting because `vim.filetype.match()` only
does filename-based detection.

Now `get_ft_from_filename()` first checks if a buffer already exists for
the file and uses its detected filetype. This requires knowing the repo
root to construct the full path, so:

- `parse_buffer()` reads `b:diffs_repo_root` or `b:git_dir` (fugitive)
- `commands.lua` sets `b:diffs_repo_root` on diff buffers it creates

Co-authored-by: phanen <phanen@qq.com>

lua/diffs/parser.lua

@@ -14,8 +14,21 @@ local M = {}
 local dbg = require('diffs.log').dbg
 
 ---@param filename string
+---@param repo_root string?
 ---@return string?
-local function get_ft_from_filename(filename)
+local function get_ft_from_filename(filename, repo_root)
+  if repo_root then
+    local full_path = vim.fs.joinpath(repo_root, filename)
+    local buf = vim.fn.bufnr(full_path)
+    if buf ~= -1 then
+      local ft = vim.api.nvim_get_option_value('filetype', { buf = buf })
+      if ft and ft ~= '' then
+        dbg('filetype from existing buffer %d: %s', buf, ft)
+        return ft
+      end
+    end
+  end
+
   local ft = vim.filetype.match({ filename = filename })
   if not ft then
     dbg('no filetype for: %s', filename)
@@ -39,10 +52,27 @@ local function get_lang_from_ft(ft)
   return nil
 end
 
+---@param bufnr integer
+---@return string?
+local function get_repo_root(bufnr)
+  local ok, repo_root = pcall(vim.api.nvim_buf_get_var, bufnr, 'diffs_repo_root')
+  if ok and repo_root then
+    return repo_root
+  end
+
+  local ok2, git_dir = pcall(vim.api.nvim_buf_get_var, bufnr, 'git_dir')
+  if ok2 and git_dir then
+    return vim.fn.fnamemodify(git_dir, ':h')
+  end
+
+  return nil
+end
+
 ---@param bufnr integer
 ---@return diffs.Hunk[]
 function M.parse_buffer(bufnr)
   local lines = vim.api.nvim_buf_get_lines(bufnr, 0, -1, false)
+  local repo_root = get_repo_root(bufnr)
   ---@type diffs.Hunk[]
   local hunks = {}
 
@@ -95,7 +125,7 @@ function M.parse_buffer(bufnr)
     if filename then
       flush_hunk()
       current_filename = filename
-      current_ft = get_ft_from_filename(filename)
+      current_ft = get_ft_from_filename(filename, repo_root)
       current_lang = current_ft and get_lang_from_ft(current_ft) or nil
       if current_lang then
         dbg('file: %s -> lang: %s', filename, current_lang)