t3404: test two "preserve merges with -p" cases

t3404: test two "preserve merges with -p" cases

There are two cases for preserving merges:

  1. The merge base is outside the trunk that is to be rebased.
     Then commits of those other parents must not be picked.

  2. The merge base is inside the trunk that is to be rebased.
     Then all the commits related to that merge must be picked
     and the merge must be redone.

The "preserve merges with -p" test case tested for case 1 only.
This patch adds case 2.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>