v3.20.0

feat(coverage): the panel clones through the same engine as the command

The panel still had its own sequential clone loop while `polygit clone` used
the bounded one, so the two could drift on concurrency, skip reasons and
destination — three things it is not obvious have drifted until a clone
behaves differently depending on which surface started it. They are now one
engine, and git::clone_repo, which nothing else called, is gone rather than
left as a second way to do it.

The panel also discovers the new clones into the repo list when the run
finishes. Until now it only recomputed coverage, so a repo you had just cloned
was invisible in the list it was cloned into until a restart.

Ships as v3.20.0.