To aid debuggability, when we find a commit that isn't signed
by our expected key, output a specific error message with the
key.
(And then add code to switch to just printing the count beyond 3
because the test suite injects 100 keys and hopefully no one
ever actually does that)
I'm thinking about adding an implementation of ed25519 signatures
with OpenSSL (so we can ship the feature with Fedora CoreOS
without requiring an additional library) and in preparation for
that it's essential that we validate that libsodium-generated
signatures and OpenSSL-generated signatures are compatible.
I don't know if they are yet actually, but the goal of this
new test is to add a pre-generated repository with a signed
commit generated by libsodium.
This will catch if e.g. there's ever a change in libsodium,
or if existing libsodium implementation versions (e.g. the
one in Debian) might differ from what we ship here.