add fish function 'ffsum' to compute a control sum of video stream

fish.nix

@@ -79,6 +79,9 @@
       gitentr.body = ''
         ${pkgs.git}/bin/git ls-tree -r HEAD --name-only | ${pkgs.entr}/bin/entr $argv
       '';
+      ffsum.body = ''
+        ${pkgs.ffmpeg}/bin/ffmpeg -hide_banner -v error $argv[..-2] -i $argv[-1] -map 0:v -f md5 -
+      '';
     };
 
     shellAliases = {