excluded_1(): support exclude files in index

Index does not really have "directories", attempts to match "foo/"
against index will fail unless someone tries to reconstruct directories
from a list of file.

Observing that dtype in this function can never be NULL (otherwise
it would segfault), dtype NULL will be used to say "hey.. you are
matching against index" and behave properly.

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

dir.c

@@ -349,6 +349,12 @@ static int excluded_1(const char *pathname,
 			int to_exclude = x->to_exclude;
 
 			if (x->flags & EXC_FLAG_MUSTBEDIR) {
+				if (!dtype) {
+					if (!prefixcmp(pathname, exclude))
+						return to_exclude;
+					else
+						continue;
+				}
 				if (*dtype == DT_UNKNOWN)
 					*dtype = get_dtype(NULL, pathname, pathlen);
 				if (*dtype != DT_DIR)