`git status` output is very misleading after a merge on a "detached HEAD"

[Enis Bayramoğlu <enis@xxxxxxxxxxxxxxxxx>]

I've encountered a very misleading output from `git status`. Here's a
sequence of events that demonstrates the issue:

$ git --version
git version 2.12.0

$ git checkout origin/master

$ git status
HEAD detached from origin/master
nothing to commit, working directory clean

$ git merge --ff f3515b749be861b57fc70c2341c1234eeb0d5b87

$ git status
HEAD detached from origin/master
nothing to commit, working directory clean

$ git rev-parse origin/master
e1dc1baaadee0f1aef2d5c45d068306025d11f67

$ git rev-parse HEAD
786cb6dd09897e0950a2bdc971f0665a059efd33

I think it's extremely misleading that `git status` simply reports
"HEAD detached from origin/master" while this simply happens to be a
mildly relevant fact about some past state.

Thanks and regards