Disable or remove the "one file at a time" checkbox when there's only one file

The following discussion from !47726 (merged) should be addressed:

• @svedova started a discussion: (+3 comments)

  question @thomasrandolph when there is only one file in the diff, clicking the checkbox seems to have no effect and it might be confusing for the user. I know this is a UX question, but do you think it should be disabled/hidden if there is only one file in the MR?