Probe git for the handshake like the other renderers, not for a record.
git announces itself for exactly the formats it describes, so asked with
the renderer's own arguments the handshake already answers the question,
and on empty input at that. Looking for a record was over-specified and
made git the one renderer probed differently for no reason.
The two probes that now both stand a diff up from two empty temp files
share that setup.