redo.py: report when you're trying to rebuild a static file.

In redo-ifchange, this might be a good idea, since you might just want to
set a dependency on it, so we won't say anything from inside builder.py.
But if you're calling redo.py, that means you expect it to be rebuilt, since
there's no other reason to try.  So print a warning.

(This is what make does, more or less.)

state.py

@@ -276,6 +276,10 @@ class File(object):
             # a "unique identifier" stamp for a regular file
             return str((st.st_ctime, st.st_mtime, st.st_size, st.st_ino))
 
+    def nicename(self):
+        return relpath(os.path.join(vars.BASE, self.name), vars.STARTDIR)
+
+
 
 # FIXME: I really want to use fcntl F_SETLK, F_SETLKW, etc here.  But python
 # doesn't do the lockdata structure in a portable way, so we have to use