diff: add --ita-[in]visible-in-index

The option --ita-invisible-in-index exposes the "ita_invisible_in_index"
diff flag to outside to allow easier experimentation with this new mode.
The "plan" is to make --ita-invisible-in-index default to keep consistent
behavior with 'status' and 'commit', but a bunch other commands like
'apply', 'merge', 'reset'.... need to be taken into consideration as well.

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

Documentation/diff-options.txt

@@ -570,5 +570,13 @@ endif::git-format-patch[]
 --line-prefix=<prefix>::
 	Prepend an additional prefix to every line of output.
 
+--ita-invisible-in-index::
+	By default entries added by "git add -N" appear as an existing
+	empty file in "git diff" and a new file in "git diff --cached".
+	This option makes the entry appear as a new file in "git diff"
+	and non-existent in "git diff --cached". This option could be
+	reverted with `--ita-visible-in-index`. Both options are
+	experimental and could be removed in future.
+
 For more detailed explanation on these common options, see also
 linkgit:gitdiffcore[7].

diff.c

@@ -3951,6 +3951,10 @@ int diff_opt_parse(struct diff_options *options,
 		return parse_submodule_opt(options, arg);
 	else if (skip_prefix(arg, "--ws-error-highlight=", &arg))
 		return parse_ws_error_highlight(options, arg);
+	else if (!strcmp(arg, "--ita-invisible-in-index"))
+		options->ita_invisible_in_index = 1;
+	else if (!strcmp(arg, "--ita-visible-in-index"))
+		options->ita_invisible_in_index = 0;
 
 	/* misc options */
 	else if (!strcmp(arg, "-z"))

t/t2203-add-intent.sh

@@ -57,7 +57,9 @@ test_expect_success 'i-t-a entry is simply ignored' '
 	git add -N nitfol &&
 	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 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 -- nitfol | wc -l) = 1
 '
 
 test_expect_success 'can commit with an unrelated i-t-a entry in index' '