git-commit bug (was Re: Editing the root commit)

Chris Webb [off-list ref] writes:

PS Whilst experimenting, I also noticed a (presumably unintentional)
behaviour:

  $ git init .
  Initialized empty Git repository in /tmp/foo/.git/
  $ git checkout --detach
  $ touch bar
  $ git add bar
  $ git commit -m test
  [(null) (root-commit) 17b5bf9] test
   0 files changed
   create mode 100644 bar
  $ ls .git/refs/heads/
  (null)
  $

Here we've created a branch with the strange name '(null)' instead of
actually detaching, or refusing to detach because we're on an unborn
branch.

This was introduced by abe199808c, which is intended to allow

  git init . && git checkout --orphan newbranch

but presumably wasn't also meant to enable

  git checkout --orphan foo
  git checkout --detach

This leads to a printf("%s", NULL) and thus

  $ git symbolic-ref HEAD
  refs/heads/(null)

I've followed up to this message with a patch including a test to catch this
in future.

Best wishes,

Chris.