Native checkout in the runner: clone from the git edge, not through g1t.sh and the egress proxy

Follows #225 and needs the git edge item. actions/checkout runs as the real JS action and clones from https://g1t.sh over the public internet; a brokered job's traffic also goes through the runner Worker's egress handler, which buffers answers up to 8 MB. Do for checkout what upload-artifact and download-artifact already have: a native implementation that fetches from the repository's git edge directly, with the same inputs and results as actions/checkout.