diff: ignore --ita-[in]visible-in-index when diffing worktree-to-tree

This option is supposed to fix the diff of "diff-files" (not reporting
ita entries as new files) and "diff-index --cached <tree>" (showing ita
entries as present in the index with empty content) but not
"diff-index <tree>".

When --ita-invisible-in-index is set on "git diff-index <tree>",
unpack_trees() will eventually call oneway_diff() on the ita entry
with the same code flow as "diff-index --cached <tree>". We want to
ignore the ita entry for "diff-index --cached <tree>" but not
"diff-index <tree>" since the latter will examine and produce a diff
based on worktree entry's (real) content, not ita index entry's
(empty) content.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff-lib.c

@@ -389,8 +389,12 @@ static void do_oneway_diff(struct unpack_trees_options *o,
 	struct rev_info *revs = o->unpack_data;
 	int match_missing, cached;
 
-	/* i-t-a entries do not actually exist in the index */
-	if (revs->diffopt.ita_invisible_in_index &&
+	/*
+	 * i-t-a entries do not actually exist in the index (if we're
+	 * looking at its content)
+	 */
+	if (o->index_only &&
+	    revs->diffopt.ita_invisible_in_index &&
 	    idx && ce_intent_to_add(idx)) {
 		idx = NULL;
 		if (!tree)

t/t2203-add-intent.sh

@@ -70,7 +70,7 @@ test_expect_success 'i-t-a entry is simply ignored' '
 	git commit -m second &&
 	test $(git ls-tree HEAD -- nitfol | wc -l) = 0 &&
 	test $(git diff --name-only HEAD -- nitfol | wc -l) = 1 &&
-	test $(git diff --name-only --ita-invisible-in-index HEAD -- nitfol | wc -l) = 0 &&
+	test $(git diff --name-only --ita-invisible-in-index HEAD -- nitfol | wc -l) = 1 &&
 	test $(git diff --name-only --ita-invisible-in-index -- nitfol | wc -l) = 1
 '