Merge branch 'nd/cache-tree-ita'

Merge branch 'nd/cache-tree-ita'

"git add -N dir/file && git write-tree" produced an incorrect tree
when there are other paths in the same directory that sorts after
"file".

* nd/cache-tree-ita:
  cache-tree: do not generate empty trees as a result of all i-t-a subentries
  cache-tree.c: fix i-t-a entry skipping directory updates sometimes
  test-lib.sh: introduce and use $EMPTY_BLOB
  test-lib.sh: introduce and use $EMPTY_TREE