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

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

diff --cc wt-status.c
Simple merge