When the magic-proxy script could not find a valid InRelease file for the
configured timestamp, it would fall back to serving the canonical version
of it. This meant that builds would succeed, even though snap-shotting the
repository failed.
This update makes the script return HTTP 404 when an InRelease by-hash
link for a given combination of mirror, suite and timestamp cannot be
found.
When the REPO_SNAPSHOT_STAMP variable is set, the auto/build script will attempt
to launch a transparent HTTP proxy on port 8080, and insert an iptables rule to
redirect all outgoing HTTP requests to this proxy.
The proxy, contained in the `magic-proxy` Python script, examines each request
and silently overrides those pointing to InRelease files or files that are
listed in InRelease files. It will instead provide the contents of the requested
file as it was at REPO_SNAPSHOT_STAMP, by downloading the corresponding asset
"by hash".
