t3418: test `rebase -r` with merge strategies

t3418: test `rebase -r` with merge strategies

There is a test case in this script that verifies that `git rebase
--preserve-merges` works all right with non-default merge strategies or
non-default merge strategy options.

Now that `git rebase --rebase-merges` learned about merge strategies,
let's copy-edit this test case to verify that that works as intended,
too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>