If a depends on b which depends on a, redo would just freeze. Now it
aborts with a somewhat helpful error message.
[Updated by apenwarr for coding style and to add a test.]
We already did this in minimal/do, and redo-ifchange already did this, but
plain redo didn't. This made constructs like:
for d in *.x; do
echo "${d%.x}"
done | xargs redo
dangerous, because if there were no matching files, we'd try to 'redo all'.
This includes a fairly detailed test of various known shell bugs from the
autoconf docs.
The idea here is that if redo works on your system, you should be able to
rely on a *good* shell to run your .do files; you shouldn't have to work
around zillions of bugs like autoconf does.
