Merge branch 'pw/rebase-abort-clean-rewritten' into maint

"git rebase --abort" used to leave refs/rewritten/ when concluding
"git rebase -r", which has been corrected.

* pw/rebase-abort-clean-rewritten:
rebase --abort/--quit: cleanup refs/rewritten
sequencer: return errors from sequencer_remove_state()
rebase: warn if state directory cannot be removed
rebase: fix a memory leak

diff --cc builtin/rebase.c
Simple merge

diff --cc sequencer.c
Simple merge

diff --cc t/t3430-rebase-merges.sh
Simple merge