Merge branch 'en/sequencer-merge-labels'

The commit labels used to explain each side of conflicted hunks
placed by the sequencer machinery have been made more readable by
humans.

* en/sequencer-merge-labels:
  sequencer: avoid garbled merge machinery messages due to commit labels

diff --cc sequencer.c
Simple merge

diff --cc t/t3404-rebase-interactive.sh
Simple merge

diff --cc t/t3507-cherry-pick-conflict.sh
Simple merge