Truncating HEAD reflog on branch move

In Git 2.11.0, using git branch -m caused an empty entry to be logged to the reflog, but it preserved the previous history of the default (HEAD) reflog. In Git 2.13.1 (and in next and pu), we write a valid entry into the reflog, but truncate it.

So for the following commands:

  git clone https://git.crustytoothpaste.net/git/bmc/homedir.git
  cd homedir
  git checkout cPanel
  git checkout master
  git branch -m master new

I get the following on 2.11.0:

2cbfbd5 HEAD@{0}: 2cbfbd5 HEAD@{1}: checkout: moving from cPanel to master eaf8db2 HEAD@{2}: checkout: moving from master to cPanel 2cbfbd5 HEAD@{3}: clone: from https://bmc@git.crustytoothpaste.net/git/bmc/homedir.git

and this on a recent next:

2cbfbd5 HEAD@{0}: Branch: renamed refs/heads/master to refs/heads/new

For this test, any repo will work; I just picked this one because it had two branches I could use to generate dummy reflog entries.

A colleague reported this to me as a bug. I don't see anything in the release notes about this as a desired behavior change, and it does seem undesirable to truncate the reflog this way. If this isn't intentional, I'm happy to work up a patch.