We previously assumed that redo and redo-ifchange are the same in
minimal/do's design, because it rebuilds all targets on every run, and
so there's no reason to ever build the same target more than once.
Unfortunately that's incorrect: if you run 'redo x' from two points in
a single run (or even twice in the same .do file), we expect x to be
built twice. If you wanted redo to decide whether to build it the
second time, you should have used redo-ifchange.
t/102-empty/touchtest was trying to test for this. However, a
second bug in minimal/do made the test pass anyway. minimal/do would
*always* rebuild any target x that produced no output, not caring
whether it had tried to build before, whether you used redo or
redo-ifchange. And while we tested that redo would redo a file that
had been deleted, we didn't ensure that it would redo a file that was
*not* deleted, nor that redo-ifchange would *not* redo that file.
Fix both bugs in minimal/do, and make t/102-empty/touchtest cover the
missing cases.
