How do I supress the "2 files to edit" message in vimdiff?

Looking through the vim source, it looks like that message can only be suppressed when launching the executable as ex and using its -s option, or by not having a console.

Neither approach will work for diffing.

However, the message is only output if there is more than one file specified on the command line.

So let's trick it:

vim a -c "vert diffsplit b"

Which basically says "Edit file a with vim, and once a is loaded, open a vertical split with file b and diff them".