should copybara info (or a related sibling) use the same semantics migrate does?

Currently one can think there are pending changes when migrate will fail (for example when you have empty commits).
@mikelalcon referenced this in #3 when he wrote: "Ideally we should honor origin_files. At least the glob roots. Would be great if we could honor the excludes too.
Probably this involves an option to origin.changes() to return the list of files in a change too."
Maybe what we need is "expensive info" since it will cost more or "migrate --dry-run" to reflect the pending commits