[feat] Make gitsigns init function async (#2538)

* [feat] Make gitsigns init function async

This function is running git and also initializing a shell, which can be
a relatively slow operation. By leveraging the jobs api, we run the
command in background, reducing the time it takes for the buffer to be
available to the user. It also uses the list format for the job, which
allow us to bypass the shell entirely.

* performance: use uv.cwd() instead of fn.expand 

benchmarked luv's cwd and it seems to be 20x faster than the expand function

---------

Co-authored-by: Sidhanth Rathod <siduck@tutanota.com>

lua/plugins/init.lua

@@ -91,13 +91,18 @@ local default_plugins = {
       vim.api.nvim_create_autocmd({ "BufRead" }, {
         group = vim.api.nvim_create_augroup("GitSignsLazyLoad", { clear = true }),
         callback = function()
-          vim.fn.system("git -C " .. '"' .. vim.fn.expand "%:p:h" .. '"' .. " rev-parse")
+          vim.fn.jobstart({"git", "-C", vim.loop.cwd(), "rev-parse"},
-          if vim.v.shell_error == 0 then
+            {
+              on_exit = function(_, return_code)
+                if return_code == 0 then
                   vim.api.nvim_del_augroup_by_name "GitSignsLazyLoad"
                   vim.schedule(function()
                     require("lazy").load { plugins = { "gitsigns.nvim" } }
                   end)
                 end
+              end
+            }
+          )
         end,
       })
     end,