In detached HEAD, "git push remote-name" should push the refspecs in
remote.remote-name.push. Since there is no test case that checks this
behavior, add one.

This is technically true, but reading the documentation, it seems that
this should also happen when we're not in detached HEAD. Maybe write:

  "git push remote-name" (that is, with no refspec given on the command
  line) should push the refspecs in remote.remote-name.push. There is no
  test case that checks this behavior in detached HEAD, so add one.

The test itself looks OK.