Some very suspicious changes to relpath() and sname() calls.

The problem is that redo-ifchange has a different $PWD than its
sub-dependencies, so as it's chasing them down, fixing up the relative paths
totally doesn't work at all.

There's probably a much smarter fix than this, but it's too late at night to
think of it right now.

redo.py

@@ -93,7 +93,7 @@ def stamp(t):
 
 
 def _preexec(t):
-    os.environ['REDO_TARGET'] = t
+    os.environ['REDO_TARGET'] = os.path.basename(t)
     os.environ['REDO_DEPTH'] = vars.DEPTH + '  '
     dn = os.path.dirname(t)
     if dn: