watch_index is deprecated, use watch_gitdir instead (#1614)

lua/core/gitsigns.lua

@@ -44,7 +44,7 @@ M.config = function()
         noremap = true,
         buffer = true,
       },
-      watch_index = { interval = 1000 },
+      watch_gitdir = { interval = 1000 },
       sign_priority = 6,
       update_debounce = 200,
       status_formatter = nil, -- Use default