t/porting/readme.t: Get list from MANIFEST

Otherwise we end up failing because of editor or patch droppings
in Porting/.  The test was already skipping things ending with ~
but that was not sufficient.
perldiag grammar

1cd88304d70 fixed one grammatical error in this sentence.  This fixes
the other.
Alphabetise perldiag

Earlier in perldiag, it says:

    The messages are in alphabetical order, without regard to upper or
    lower-case.