CI: Limit git operations to those needed to speed up jobs

Use 'fetch' instead of 'clone' to get the most recent source code. This is faster, as it re-uses the project workspace, and falls back if it doesn't exist.

Further, limit the depth of fetching or cloning to the last ten commits.

Also, disable any git operations for all jobs, which rely exclusively on artifacts produced by former jobs, i.e. no source code at all involved.