If we're using a .do file from a parent directory, we should set $3 using
the same path prefix as $1. We were previously using just the basename,
which mostly works (since we would rename it to $1$2 eventually anyway) but
is not quite right, and you can't safely rename files across filesystems, so
it could theoretically cause problems.
Also improved t/defaults-nested to test for this behaviour.
Reported by Eric Kow.
Previously, we would only search for default*.do in the same directory in
the target; now we search parent directories as well.
Let's say we're in a/b/ and trying to build foo.o. If we find
../../default.o.do, then we'll run
cd ../..; sh default.o.do a/b/foo .o $TMPNAME
In other words, we still always chdir to the same directory as the .do file.
But now $1 might have a path in it, not just a basename.
