Merge branch 'jc/detached-head'

* jc/detached-head:
git-checkout: handle local changes sanely when detaching HEAD
git-checkout: safety check for detached HEAD checks existing refs
git-checkout: fix branch name output from the command
git-checkout: safety when coming back from the detached HEAD state.
git-checkout: rewording comments regarding detached HEAD.
git-checkout: do not warn detaching HEAD when it is already detached.
Detached HEAD (experimental)
git-branch: show detached HEAD
git-status: show detached HEAD

diff --cc cache.h
Simple merge

diff --cc path.c
Simple merge

diff --cc wt-status.c
Simple merge