sequencer: advise if skipping cherry-picked commit

sequencer: advise if skipping cherry-picked commit

Silently skipping commits when rebasing with --no-reapply-cherry-picks
(currently the default behavior) can cause user confusion. Issue
warnings when this happens, as well as advice on how to preserve the
skipped commits.

These warnings and advice are displayed only when using the (default)
"merge" rebase backend.

Update the git-rebase docs to mention the warnings and advice.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>