--snapshot-format is the standard output format. -s is the prefix, not the actual filename.
That is, for MD5 one should do:
./glretrace --snapshot-format=MD5 -s - app.trace
f8da408 retrace: Make --snapshot-format=MD5 work as documented.
retrace/retrace_main.cpp | 19 +++++++++++++------
1 file changed, 13 insertions(+), 6 deletions(-)
Upstream: github.com