Don't crash on targets in directories that don't exist yet.

The reason we'd crash is that we tried to pre-create a file called
$target.redo.tmp, which wouldn't work because the directory containing
$target didn't exist.

We now try to generate a smarter filename by using the innermost directory
of target that *does* exist.  It's a little messy, but the idea is to make
sure we won't have to rename() across a filesystem boundary if, for example,
there's a mounted filesystem in the middle of the hierarchy somewhere.

clean.do

@@ -1,7 +1,7 @@
 rm -rf t/.redo redo-sh
 if [ -e .do_built ]; then
 	while read x; do
-		rm -f "$x"
+		[ -d "$x" ] || rm -f "$x"
 	done <.do_built
 fi
 [ -z "$DO_BUILT" ] && rm -rf .do_built .do_built.dir