Merge branch 'mm/detach-at-HEAD-reflog'

Merge branch 'mm/detach-at-HEAD-reflog'

After "git checkout --detach", "git status" reported a fairly
useless "HEAD detached at HEAD", instead of saying at which exact
commit.

* mm/detach-at-HEAD-reflog:
  status: don't say 'HEAD detached at HEAD'
  t3203: test 'detached at' after checkout --detach