rebase: support --trailer

rebase: support --trailer

Add a new --trailer=<trailer> option to git rebase to append trailer
lines to each rewritten commit message (merge backend only).

Because the apply backend does not provide a commit-message filter,
reject --trailer when --apply is in effect and require the merge backend
instead.

This option implies --force-rebase so that fast-forwarded commits are
also rewritten. Validate trailer arguments early to avoid starting an
interactive rebase with invalid input.

Add integration tests covering error paths and trailer insertion across
non-interactive and interactive rebases.

Signed-off-by: Li Chen <me@linux.beauty>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>