rebase-i-p: fix 'no squashing merges' tripping up non-merges

rebase-i-p: fix 'no squashing merges' tripping up non-merges

Also only check out the first parent if this commit if not a squash--if it is a
squash, we want to explicitly ignore the parent and leave the wc as is, as
cherry-pick will apply the squash on top of it.

Signed-off-by: Stephen Haberman <stephen@exigencecorp.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>