Tweak scripts/regen-from-raw to work on dash
Tweak scripts/regen-from-raw to work on dash

The behavior of dash and bash are different in test ... -ot
when the first file does not exist.  Tweak the code so it
works on both dash and bash.

Signed-off-by: David A. Wheeler <9ae72d22d8b894b865a7a496af4fab6320e6abb2@dwheeler.com>

metamath/set.mm,metamath/set.mm,metamath/set.mm