Hide comparison of checkout with previous slot build in '/ci-test' jobs

Currently we show in checkout logs which commits have bee removed/added since the previous build.

While it make sense for regular slots, it is useless for slots triggered by the '/ci-test' command, where the previous build could be completely unrelated.

What we need is:

• flag the slot as special via metadata (#24 (closed))
• extend checkout logic to avoid the comparison for the special slots