builder.py: refresh the File object after obtaining the lock.

We need to create the File object to get its f.id, then lock that id.
During that gap, another instance of redo may have modified the file or
its state data, so we have to refresh it.

This fixes 'redo -j10 t/stress'.

t/all.do

@@ -29,6 +29,3 @@ sed 's/\.do$//' | {
         redo "$d"
     done
 }
-
-# if all that worked run a repeated stress test to look for races
-redo stress