Undocumented change in `git branch -M` behavior

Hello,

Hopefully, I've got this right -- I noticed a change in behavior in git with Ubuntu 17.10, which recently got 2.14.1. Specifically, that when in an orphaned branch, -M ends up moving HEAD to the new branch name, clobbering the working tree. As far as I know, from the manpages, orphaned branches are still supported and should work?

I think an example will demonstrate more than words (the following are done in LXD containers, hence the root user):

# git --version
git version 2.14.1
# mkdir test && cd test && git init .
Initialized empty Git repository in /root/test/.git/
# git checkout -b a
Switched to a new branch 'a'
# touch testfile && git add testfile && git commit -m 'initial commit'
[a (root-commit) 6061193] initial commit
 Committer: root <root@precious-magpie.lxd>
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 testfile
# git checkout --orphan master
Switched to a new branch 'master'
# git status
On branch master

No commits yet

Changes to be committed:
  (use "git rm --cached <file>..." to unstage)

        new file:   testfile

# git reset --hard && git status On branch master

No commits yet

nothing to commit (create/copy files and use "git add" to track)
# git branch -M a b
# git status
On branch b
Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

        deleted:    testfile

This is very unexpected. I force-renamed a branch I wasn't currently checked out to and now I'm checked out to it *and* I have staged file removals (I think what is effectively happening is my current working directory (empty) is being staged into the new branch, but I'm not 100%).

For comparision, on 17.04:

# git --version
git version 2.11.0
# mkdir test && cd test && git init .
Initialized empty Git repository in /root/test/.git/
# git checkout -b a
Switched to a new branch 'a'
# touch testfile && git add testfile && git commit -m 'initial commit'
[a (root-commit) f8d0d53] initial commit
 Committer: root <root@honest-sturgeon.lxd>
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 testfile
# git checkout --orphan master
Switched to a new branch 'master'
# git status
On branch master

No commits yet

Changes to be committed:
  (use "git rm --cached <file>..." to unstage)

        new file:   testfile

# git reset --hard && git status On branch master

No commits yet

nothing to commit (create/copy files and use "git add" to track) # git branch -M a b # git status On branch master

Initial commit

nothing to commit (create/copy files and use "git add" to track)

This is what I expect to see, the branch rename has no effect on HEAD.

I haven't yet bisected this (but I can if necessary). My initial suspicion is https://github.com/git/git/commit/70999e9ceca47e03b8900bfb310b2f804125811e#diff-d18f86ea14e2f1e5bff391b2e54438cb where a comparison between the oldname of the branch and HEAD was performed before attempting to move HEAD (so that HEAD followed to the new branch name, I believe). That change was dropped, though and perhaps the new check in replace_each_worktree_head_symref of

        strcmp(oldref, worktrees[i]->head_ref)

does not work for orphaned branches? I am unfamiliar with all the details of the git internals, so please correct me if I'm wrong!

Thanks, Nish