git svn: stop using `rebase --preserve-merges`

git svn: stop using `rebase --preserve-merges`

We deprecated `--preserve-merges` in favor of `--rebase-merges`; Let's
reflect that in `git svn`.

Note: Even when the user asks for `--preserve-merges`, we now silently
pass `--rebase-merges` to `git rebase` instead. Technically, this is a
change of behavior. But practically, `git svn` only ever asks for a
non-interactive rebase, and `--preserve-merges` and `--rebase-merges`
are on par with regard to that.

Signed-off-by: Johannes Schindelin <redacted>
Acked-by: Eric Wong <redacted>
Signed-off-by: Junio C Hamano <redacted>