Add create-manifest option

When working on features that span multiple repositories, e.g. when changing Core APIs used in one or more plugins, one needs to create new manifests that are largely based on `master.json`.

Currently one has to manually edit the manifest file to change branch names or add a new repository.

Create a new command that takes an existing manifest (e.g. the currently checked out) and creates a new one, based on the current branches.