TODO


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Add --include option (opposite of --exclude).

Look into sdiff improvement here:
http://www.pkix.net/~chuck/sdiff2.diff

Propagate stderr from subprocess so that diff3 does
a better job of explaining _why_:
  > $ diff3 /bin/sh /bin/false /bin/mv
  > diff3: subsidiary program 'diff' failed (exit status 2)