fast-export: do not export negative refs
When calling `git fast-export master..next` we want to export
refs/heads/next, but not refs/heads/master.
Currently this is not a problem, because negative refs' commits
are never shown. In the next commit this will be changed in order
to make sure that 'master..master' does export master. I.e. even
refs whose commits are not shown are exported.
Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com>