`git prole convert` now creates a tempdir in the parent of the
determined destination of the converted repo.
This prevents some errors when the /tmp directory is on a different
filesystem, such as a tmpfs.

The tempdir will be deleted if and only if it is empty after conversion.
This should always be the case and is to prevent any data loss.

Fixes issue 9999years#96

* `fs::read_dir` skips entries for the current and parent directories, so
  we want it to be completely empty to delete the temporary directory.

* Add a local `fs::read_dir` wrapper for `fs_err::read_dir` to match the
  others in that module.

* Show a warning, not an info message, if the tempdir isn't empty.

* Include the paths in the tempdir in the warning message.