Although I expect this is rather rare, some people may want to build in
a read-write subdir of a read-only tree. Other than some confusing
error reporting, this works fine in redo after the recent changes to
temp file handling, but let's add a test to make sure it stays that
way. The test found a bug in minimal/do, so let's fix that.
Reported-by: Jeff Stearns <jeff.stearns@gmail.com>
