diff: Implement virt-diff --long-options.

The code was only partially implemented so the option did
not function.

diff/diff.c

@@ -166,6 +166,7 @@ main (int argc, char *argv[])
     { "format", 2, 0, 0 },
     { "help", 0, 0, HELP_OPTION },
     { "human-readable", 0, 0, 'h' },
+    { "long-options", 0, 0, 0 },
     { "keys-from-stdin", 0, 0, 0 },
     { "short-options", 0, 0, 0 },
     { "time", 0, 0, 0 },