redo-ifchange already does this, and then calls redo.py, so it's redundant.
The only weird case is if a .do file calls 'redo' instead of
'redo-ifchange'. I'm not sure what exactly is supposed to happen here.
test.do does this, for example, and it works, but only because we always do
'redo test' and nobody depends on 'redo-ifchange test'; if they did, the
dependencies would be off, I guess. But maybe it's meaningless to do that
anyway.
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