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

diff --cc t/t1011-read-tree-sparse-checkout.sh
Simple merge

diff --cc t/t7063-status-untracked-cache.sh
Simple merge

diff --cc t/t7508-status.sh
Simple merge

diff --cc t/test-lib.sh
Simple merge