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>