Add M.refresh_signs() function

lua/jj_mini_diff/init.lua

@@ -123,6 +123,11 @@ local function _is_jj_repo()
   return false
 end
 
+function M.refresh_signs()
+  if _is_jj_repo() then
+    _place_signs_in_buffer()
+  end
+end
 
 function M.setup(opts)
   opts = opts or {}