upload: Keep a review stacked on its relative branch's tip

[jerry-skydio]

A relative branch is someone else's work that a review is stacked on, so the
review has to sit exactly on the commit it will be uploaded to, the same rule a
relative series already follows. Rebase detection instead asked whether the
remote base is an ancestor of the local one, which is right for a base branch
(don't re-push every time master moves) but always true for a relative branch
that has moved forward. Upload retargeted the pr on the forge and then skipped
the push as a rebase, leaving the review based on an old tip and showing the
relative branch's commits as its own.

A relative branch review is now either nochange, when it is already on the tip,
or pushed. This means such a review re-pushes whenever the relative branch
moves, even without --rebase.

[jerry-skydio]

Reviews in this chain:
└#304 upload: Keep a review stacked on its relative branch's tip

[jerry-skydio]

#	head	base	diff	date	summary
0	330d846d	137d0090	diff	Sep 25 17:57 PM	2 files changed, 65 insertions(+), 5 deletions(-)