Improve building bug-fix releases

[zdavis]

Currently, if we want to build a patch release, we do this:

./exe/manifold-tools build v5.1.1 --branch=v5.x
./exe/manifold-tools publish v5.1.1 --branch=v5.x

This mostly works, but there are still a few manual cleanup steps that could be automated:

• When manifold-tools opens the PR against ManifoldScholar/Manifold as part of publishing, the PR is against master instead of against the branch that's being built.
• When manifold-tools tags the ManifoldScholar/Manifold repo, it checks out master first and tags master. The tag should be on the HEAD of the branch specified by --branch
• When manifold-tools generates the changelog, it includes all closed PRs to date, not just the ones that appear in the branch that's being built and published.