checkout.c: unstage empty deleted ita files

checkout.c: unstage empty deleted ita files

It is possible to delete a committed file from the index and then add it
as intent-to-add. After `git checkout HEAD <pathspec>`, the file should
be identical in the index and HEAD. The command already works correctly
if the file has contents in HEAD. This patch provides the desired
behavior even when the file is empty in HEAD.

`git checkout HEAD <pathspec>` calls tree.c:read_tree_1(), with fn
pointing to checkout.c:update_some(). update_some() creates a new cache
entry but discards it when its mode and oid match those of the old
entry. A cache entry for an ita file and a cache entry for an empty file
have the same oid. Therefore, an empty deleted ita file previously
passed both of these checks, and the new entry was discarded, so the
file remained unchanged in the index. After this fix, if the file is
marked as ita in the cache, then we avoid discarding the new entry and
add the new entry to the cache instead.

This change should not affect newly added ita files. For those, inside
tree.c:read_tree_1(), tree_entry_interesting() returns
entry_not_interesting, so fn is never called.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Varun Naik <vcnaik94@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>