Publish the libraries a few at a time - #351

Once the tests are done, the run publishes one library at a time, and
what that costs is not the copying but waiting for the far side: each
library asks rsync four times over ssh, and the one that matters lists
a remote directory of tens of thousands of files to honour
`--delete-excluded`.

Three things were paid for per library and are now paid for once:

| What                               | Before       | After        |
| ---------------------------------- | ------------ | ------------ |
| git log of this repository         | once per lib | once         |
| git log of the reference files     | once per lib | once per dir |
| rsync connections to make the dirs | 3            | 1            |

The directories are made in one connection by telling rsync -R where
the relative part of the source path starts, so `emptydir/./<lib>/files`
creates the branch, the library and its files directory at once instead
of one rsync per level.

The libraries are then published four at a time. Not more: each one is
an ssh connection of its own, and sshd allows ten unauthenticated
connections by default.

The loop body became a function, which also stops it shadowing the
`stats` dictionary of the run with a list of its own.

Assisted-by: Claude Opus 5