Bugfix for the recently graduated "git rebase --rebase-merges".

* js/rebase-r-and-merge-head:
  status: rebase and merge can be in progress at the same time
  built-in rebase --skip/--abort: clean up stale .git/<name> files
  rebase -i: include MERGE_HEAD into files to clean up
  rebase -r: do not write MERGE_HEAD unless needed
  rebase -r: demonstrate bug with conflicting merges