Do not run git diff against .git directory

This happens when running diff from `git commit` message.

Closes CopilotC-Nvim#417

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

Author: deathbeam

lua/CopilotChat/select.lua

@@ -167,6 +167,7 @@ function M.gitdiff(source, staged)
   if not dir or dir == '' then
     return nil
   end
+  dir = dir:gsub('.git$', '')
 
   local cmd = 'git -C ' .. dir .. ' diff --no-color --no-ext-diff' .. (staged and ' --staged' or '')
   local handle = io.popen(cmd)