Roughly speaking, this changes (assuming 3 upstream PRs)
git merge <upstream-commit1> <upstream-commit-2> <upstream-commit3>
into
git merge <upstream-commit3>
This is more intuitive. We're merging a single upstream revision, namely
<upstream-commit3>. The other two commits are simply parents of that one,
i.e., they're included anyway, and git merge ignores them.
(In fact, passing multiple refs looks like we're doing an octopus
merge. It's just that git recognizes the fact that everything is included
in the last ref anyway, and behaves as if only the last one had been passed.)
This commit also makes some further clean ups and improvements.
656c7cc704