rebase-i-p: do not include non-first-parent commits touching UPSTREAM

rebase-i-p: do not include non-first-parent commits touching UPSTREAM

This covers an odd boundary case found by Avi Kivity's script where a branch
coming off of UPSTREAM is merged into HEAD. Initially it show up in
UPSTREAM..HEAD, but technically UPSTREAM is not moving, the rest of head is, so
we should not need to rewrite the merge.

This adds a check saying we can keep `preserve=t` if `p=UPSTREAM`...unless this
is the first first-parent commit in our UPSTREAM..HEAD rev-list, which could
very well point to UPSTREAM, but we still need to consider it as rewritten so we
start pulling in the rest of the UPSTREAM..HEAD commits that point to it.

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