refactor(context): replace io.popen with vim.system

Replace synchronous io.popen git diff call with asynchronous vim.system in
context.lua. Add async scheduler to token fetching and introduce a new utils
helper for system calls.

This change improves overall plugin performance by making git operations
asynchronous and following Neovim's best practices for system calls.

lua/CopilotChat/context.lua

@@ -353,25 +353,23 @@ end
 function M.gitdiff(type, winnr)
   type = type or 'unstaged'
   local cwd = utils.win_cwd(winnr)
-  local cmd = 'git -C ' .. cwd .. ' diff --no-color --no-ext-diff'
+  local cmd = {
+    'git',
+    '-C',
+    cwd,
+    'diff',
+    '--no-color',
+    '--no-ext-diff',
+  }
 
   if type == 'staged' then
-    cmd = cmd .. ' --staged'
-  end
-
-  local handle = io.popen(cmd)
-  if not handle then
-    return nil
+    table.insert(cmd, '--staged')
   end
 
-  local result = handle:read('*a')
-  handle:close()
-  if not result or result == '' then
-    return nil
-  end
+  local out = utils.system(cmd)
 
   return {
-    content = result,
+    content = out.stdout,
     filename = 'git_diff_' .. type,
     filetype = 'diff',
   }

lua/CopilotChat/copilot.lua

@@ -18,6 +18,7 @@
 ---@field model string?
 ---@field chunk_size number?
 
+local async = require('plenary.async')
 local log = require('plenary.log')
 local prompts = require('CopilotChat.prompts')
 local tiktoken = require('CopilotChat.tiktoken')
@@ -50,6 +51,8 @@ local VERSION_HEADERS = {
 --- Get the github oauth cached token
 ---@return string|nil
 local function get_cached_token()
+  async.util.scheduler()
+
   -- loading token from the environment only in GitHub Codespaces
   local token = os.getenv('GITHUB_TOKEN')
   local codespaces = os.getenv('CODESPACES')

lua/CopilotChat/utils.lua

@@ -305,4 +305,10 @@ M.read_file = function(path)
   return data
 end
 
+--- Call a system command
+---@param cmd table The command
+M.system = async.wrap(function(cmd, callback)
+  vim.system(cmd, { text = true }, callback)
+end, 2)
+
 return M