cmd-diff: default --from to N-1 build

This way if it's not specified some sane value is used.

Author: dustymabe

src/cmd-diff

@@ -78,7 +78,7 @@ def main():
         args.diff_from = builds.get_previous()
         args.diff_to = latest_build
     elif args.diff_from is None:
-        args.diff_from = latest_build
+        args.diff_from = builds.get_previous()
     elif args.diff_to is None:
         args.diff_to = latest_build