minimal/do: didn't work correctly with multi-level auto dir creation.

If we tried to build target a/b/c/d and a/b/c didn't exist yet, we
would correctly name the temp file something like a__b/c__d.tmp.  But
if a/b didn't exist yet, we named the temp file a/b__c/d.tmp, which
didn't work.  Instead, name it a/b__c__d.tmp as expected.

t/250-makedir/autosubdir/all.do

@@ -1,3 +1,11 @@
-rm -rf sub.tmp
+rm -rf sub.tmp sub2.tmp sub3.tmp
+
 redo-ifchange sub.tmp/test.txt
 [ -e sub.tmp/test.txt ] || exit 96
+
+redo-ifchange sub2.tmp/a/b/c/test.txt
+[ -e sub2.tmp/a/b/c/test.txt ] || exit 97
+
+mkdir -p sub3.tmp/a
+redo-ifchange sub3.tmp/a/b/c/test.txt
+[ -e sub2.tmp/a/b/c/test.txt ] || exit 98