Stop using meta mode for the initial checkout, use size_limit instead

Previously, we used meta mode to prepare a clean checkout, and them switched the meta mode off immediately afterwards. However, OBS 2.10 no longer supports this behaviour, which was always a hack: at the "validate" step of the commit process, it returns the _meta file as part of the filelist, but as this file is special, it’s not present in the local filelist, resulting in an error message, failing upload and a successful error code (!) being returned:

Transmitting file data There is no sha256 sum for file _meta.
This could be due to an outdated working copy.
Please update your working copy with osc update and
commit again afterwards.

Instead of this, check out the package in the normal mode, skipping the downloads, and re-add all files from scratch.