chore(setup): make setup idempotent (#1340)

Co-authored-by: Alexander Courtis <alex@courtis.org>

lua/nvim-tree/git/init.lua

@@ -134,6 +134,11 @@ function M.load_project_status(cwd)
   return M.projects[project_root]
 end
 
+function M.purge_state()
+  M.projects = {}
+  M.cwd_to_project_root = {}
+end
+
 function M.setup(opts)
   M.config = opts.git
   M.config.watcher = opts.filesystem_watchers