:Gmsg fails on empty commits

[ain]

Steps to reproduce

1. Push empty commit with :Grebuild
2. Try updating commit message with :Gmsg

What happens

You are currently rebasing branch '*****' on '********'.
  (all conflicts fixed: run "git rebase --continue")

No changes
You asked to amend the most recent commit, but doing so would make
it empty. You can repeat your command with --allow-empty, or you can
remove the commit entirely with "git reset HEAD^".

What should happen

It should be possible to change Rebuild commit message.