Merge branch 'nd/cache-tree-ita' into maint

"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