Merge branch 'dd/rebase-merge-reserves-onto-label'

The logic to avoid duplicate label names generated by "git rebase
--rebase-merges" forgot that the machinery itself uses "onto" as a
label name, which must be avoided by auto-generated labels, which
has been corrected.

* dd/rebase-merge-reserves-onto-label:
  sequencer: handle rebase-merges for "onto" message

diff --cc sequencer.c
Simple merge

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