Because the two programs use separate state databases, it helps if we
clean up some temp files between runs. Otherwise they might think you
created some targets "by hand" and refuse to rebuild them.
I think this aligns better with how redo works. Otherwise, if a.do
creates a as a symlink, then changes to the symlink's *target* will
change a's stat/stamp information without re-running a.do, which looks
to redo like you modified a by hand, which causes it to stop running
a.do altogether.
With this change, modifications to a's target are okay, but they don't
trigger any redo dependency changes. If you want that, then a.do
should redo-ifchange on its symlink target explicitly.
