Merge branch 'kh/sequencer-comment-char'

The sequencer failed to honor core.commentString in some places.

* kh/sequencer-comment-char:
  sequencer: comment commit messages properly
  sequencer: comment `--reference` subject line properly
  sequencer: comment checked-out branch properly

diff --cc sequencer.c
Simple merge

diff --cc t/t3400-rebase.sh
Simple merge

diff --cc t/t3437-rebase-fixup-options.sh
Simple merge

diff --cc t/t3501-revert-cherry-pick.sh
Simple merge