commit: fix empty commit creation when there's no changes but ita entries

If i-t-a entries are present and there is no change between the index
and HEAD i-t-a entries, index_differs_from() still returns "dirty, new
entries" (aka, the resulting commit is not empty), but cache-tree will
skip i-t-a entries and produce the exact same tree of current
commit.

index_differs_from() is supposed to catch this so we can abort
git-commit (unless --no-empty is specified). Update it to optionally
ignore i-t-a entries when doing a diff between the index and HEAD so
that it would return "no change" in this case and abort commit.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/diff.h b/diff.h
index ce3535e..94422e1 100644
--- a/diff.h
+++ b/diff.h
@@ -357,7 +357,7 @@ extern int diff_result_code(struct diff_options *, int);
 
 extern void diff_no_index(struct rev_info *, int, const char **);
 
-extern int index_differs_from(const char *def, int diff_flags);
+extern int index_differs_from(const char *def, int diff_flags, int ita_invisible_in_index);
 
 /*
  * Fill the contents of the filespec "df", respecting any textconv defined by