[m-rev.] diff: warn about unknown format strings

[Zoltan Somogyi]

When I added the option to warn about unknown format strings,
we didn't enable it by default because we would have got a lot of
warnings for calls whose format strings were too long to fit on
one line, and were therefore put on two lines and appended together.
Now that we can track format strings through appends, this is
no longer a concern, so this diff enables that warning for the
Mercury system.

I don't want to commit this until everyone has installed the change
to track format strings through appends (committed thursday,
IIRC), so can people please tell me whether they have done
an install since then?

Zoltan.
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