grep: make it clear i-t-a entries are ignored

The expression "!S_ISREG(ce)" covers i-t-a entries as well because
ce->ce_mode would be zero then. I could make a comment saying that, but
it's probably better just to comment with code, in case i-t-a entry
content changes in future.

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

builtin/grep.c

@@ -375,7 +375,7 @@ static int grep_cache(struct grep_opt *opt, const struct pathspec *pathspec, int
 
 	for (nr = 0; nr < active_nr; nr++) {
 		const struct cache_entry *ce = active_cache[nr];
-		if (!S_ISREG(ce->ce_mode))
+		if (!S_ISREG(ce->ce_mode) || ce_intent_to_add(ce))
 			continue;
 		if (!ce_path_match(ce, pathspec, NULL))
 			continue;