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

Turns out this behavior wasn't really required and there
was some preference to leave it the other way [1].

This reverts commit 9ffb64f.

[1] #4253 (comment)

Author: dustymabe

src/cmd-diff

@@ -147,7 +147,7 @@ def main():
         args.diff_from = builds.get_previous()
         args.diff_to = latest_build
     elif args.diff_from is None:
-        args.diff_from = builds.get_previous()
+        args.diff_from = latest_build
     elif args.diff_to is None:
         args.diff_to = latest_build