builder.py: correctly set $3 to include the subdir path.

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.

t/defaults-nested/default.do

@@ -1,2 +1 @@
-echo root $1 $2
-
+echo root $1 $2 "$(dirname $3)"