Merge branch 'js/rebase-r-and-merge-head'

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