diff-files: treat "i-t-a" files as "not-in-index"

The `diff-files' command and related commands which call `cmd_diff_files()',
consider the "intent-to-add" files as a part of the index when comparing the
work-tree against it. This was previously addressed in [1] and [2] by turning
the option `--ita-invisible-in-index' (introduced in [3]) on by default.

For `diff-files' (and `add -p' as a consequence) to show the i-t-a files as
as new, `ita_invisible_in_index' will be enabled by default here as well.

[1] 0231ae71d3 (diff: turn --ita-invisible-in-index on by default, 2018-05-26)
[2] 425a28e0a4 (diff-lib: allow ita entries treated as "not yet exist in
                index", 2016-10-24)
[3] b42b451919 (diff: add --ita-[in]visible-in-index, 2016-10-24)

Signed-off-by: Srinidhi Kaushik <shrinidhi.kaushik@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/diff-files.c

@@ -28,6 +28,13 @@ int cmd_diff_files(int argc, const char **argv, const char *prefix)
 	git_config(git_diff_basic_config, NULL); /* no "diff" UI options */
 	repo_init_revisions(the_repository, &rev, prefix);
 	rev.abbrev = 0;
+
+	/*
+	 * Consider "intent-to-add" files as new by default, unless
+	 * explicitly specified in the command line or anywhere else.
+	 */
+	rev.diffopt.ita_invisible_in_index = 1;
+
 	precompose_argv(argc, argv);
 
 	argc = setup_revisions(argc, argv, &rev, NULL);

t/t2203-add-intent.sh

@@ -232,7 +232,7 @@ test_expect_success 'double rename detection in status' '
 	)
 '
 
-test_expect_success 'diff-files/diff-cached shows ita as new/not-new files' '
+test_expect_success 'diff/diff-cached shows ita as new/not-new files' '
 	git reset --hard &&
 	echo new >new-ita &&
 	git add -N new-ita &&
@@ -243,6 +243,29 @@ test_expect_success 'diff-files/diff-cached shows ita as new/not-new files' '
 	test_must_be_empty actual2
 '
 
+test_expect_success 'diff-files shows i-t-a files as new files' '
+	git reset --hard &&
+	touch empty &&
+	content="foo" &&
+	echo $content >not-empty &&
+	git add -N empty not-empty &&
+	git diff-files -p >actual &&
+	hash_e=$(git hash-object empty) &&
+	hash_n=$(git hash-object not-empty) &&
+	cat >expect <<-EOF &&
+	diff --git a/empty b/empty
+	new file mode 100644
+	index 0000000..$(git rev-parse --short $hash_e)
+	diff --git a/not-empty b/not-empty
+	new file mode 100644
+	index 0000000..$(git rev-parse --short $hash_n)
+	--- /dev/null
+	+++ b/not-empty
+	@@ -0,0 +1 @@
+	+$content
+	EOF
+	test_cmp expect actual
+'
 
 test_expect_success '"diff HEAD" includes ita as new files' '
 	git reset --hard &&