git-svn: drop support for `--preserve-merges`

git-svn: drop support for `--preserve-merges`

We already passed the `--rebase-merges` option to `git rebase` instead,
now we make this move permanent.

As pointed out by Ævar Arnfjörð Bjarmason, in contrast to the
deprecation of `git rebase`'s `--preserve-merges` backend, `git svn`
only deprecated this option in v2.25.0 (because this developer missed
`git svn`'s use of that backend when deprecating the rebase backend
running up to Git v2.22).

Still, v2.25.0 has been released on January 13th, 2020. In other words,
`git svn` deprecated this option over one and a half years ago, _and_
has been redirecting to the `--rebase-merges` option during all that
time (read: `git svn rebase --preserve-merges` didn't do _precisely_
what the user asked, since v2.25.0, anyway, it fell back to pretending
that the user asked for `git svn rebase --rebase-merges` instead).

It is time to act on that deprecation and remove that option after all.

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>