A command like "hg diff b a" should print the diffs in the order specified.
Instead, it sorts the arguments, and prints the diffs in that order.

Running these commands:

    mkdir d && cd d
    touch a b
    hg init
    hg add .
    hg ci -m. .
    echo a > a; echo b > b
    hg diff b a |grep diff

I get this output:

    diff -r bd5f22546a9dfb07ffa0cb9c368ba16f00cab6cc a
    diff -r bd5f22546a9dfb07ffa0cb9c368ba16f00cab6cc b

I expected to see "b"s diffs before those of "a".