rebase: drop support for `--preserve-merges`

rebase: drop support for `--preserve-merges`

This option was deprecated in favor of `--rebase-merges` some time ago,
and now we retire it.

To assist users to transition away, we do not _actually_ remove the
option, but now we no longer implement the functionality. Instead, we
offer a helpful error message suggesting which option to use.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>