fix: fileformat autocmd bug

Fixes #1123

Author: lewis6991

lua/gitsigns/attach.lua

@@ -124,7 +124,9 @@ local setup = util.once(function()
         return
       end
       bcache:invalidate(true)
-      manager.update(buf)
+      async.run(function()
+        manager.update(buf)
+      end)
     end,
   })
 

lua/gitsigns/manager.lua

@@ -454,6 +454,7 @@ function M.schedule(bufnr, check_compare_text)
   return true
 end
 
+--- @async
 --- Ensure updates cannot be interleaved.
 --- Since updates are asynchronous we need to make sure an update isn't performed
 --- whilst another one is in progress. If this happens then schedule another