Fixed test bugs around handling corruption

The main thing to consider was how lfs_dir_fetchwith reacts to
corruption it finds and to make sure falling back to old values works
correctly.

Some of the tricky bits involved making sure we could fall back to both old
commits and old metadata blocks while still handling things like
synthetic moves correctly.

tests/test_corrupt.sh

@@ -78,11 +78,8 @@ do
     rm -rf blocks
     mkdir blocks
     ln -s /dev/zero blocks/$(printf '%x' $i)
-    echo $i 1i
     lfs_mktree
-    echo $i 2i
     lfs_chktree
-    echo $i 3i
 done
 
 echo "--- Block persistance ---"