feat: use git-dir and work-tree from environment (#142)

Passes `work-tree` and `git-dir` arguments for Lazygit.

This is very useful when using a bare git repo, so that Lazygit can still be used to manage the repo.

lua/lazygit.lua

@@ -101,7 +101,9 @@ local function lazygit(path)
     cmd = cmd .. " -ucf '" .. config_path .. "'" -- quote config_path to avoid whitespace errors
   end
 
-  if path == nil then
+  if vim.env.GIT_DIR ~= nil and vim.env.GIT_WORK_TREE ~= nil then
+    cmd = cmd .. " -w " .. vim.env.GIT_WORK_TREE .. " -g " .. vim.env.GIT_DIR
+  elseif path == nil then
     if is_symlink() then
       path = project_root_dir()
     end