Looking at the Vim 7.4.265 startup code , there is no way to suppress the message %d files to editcoming from the terminal (and therefore being visible after exiting) when called vimdiff.
I think you can always send a patch to suppress this message using the switch.
Update
I knew there would be a way to get the desired result (without writing C)!
Call Vim as vimwith one file argument. Then call :diffsplitto the second file. But from the command line through -c:
vim /path/to/first_file -c'diffsplit /path/to/second_file'
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