rebase-i: keep old parents when preserving merges

rebase-i: keep old parents when preserving merges

When "rebase -i -p" tries to preserve merges of unrelated branches, it
lost some parents:

 - When you have more than two parents, the commit in the new history
   ends up with fewer than expected number of parents and this breakage
   goes unnoticed;

 - When you are rebasing a merge with two parents and one is lost, the
   command tries to cherry-pick the original merge commit, and the command
   fails.

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