git add --intent-to-add: fix removal of cached emptiness
This uses the extended index flag mechanism introduced earlier to mark
the entries added to the index via "git add -N" with CE_INTENT_TO_ADD.
The logic to detect an "intent to add" entry for the purpose of allowing
"git rm --cached $path" is tightened to check not just for a staged empty
blob, but with the CE_INTENT_TO_ADD bit. This protects an empty blob that
was explicitly added and then modified in the work tree from being dropped
with this sequence: