Fixed the orphan test to handle logging metadata-pairs

The main issue here was that the old orphan test relied on deleting the
block that contained the most recent update. In the new design this
doesn't really work since updates get appended to metadata-pairs
incrementally.

This is fixed by instead using the truncate command on the appropriate
block. We're now passing orphan tests.

tests/test_corrupt.sh

@@ -78,8 +78,11 @@ 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 ---"