Add S_IFINVALID mode

S_IFINVALID is used to signal, that no mode information is available.
Signed-off-by: NMartin Koegler <mkoegler@auto.tuwien.ac.at>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

cache.h

@@ -24,6 +24,9 @@
 #define DTYPE(de)	DT_UNKNOWN
 #endif
 
+/* unknown mode (impossible combination S_IFIFO|S_IFCHR) */
+#define S_IFINVALID     0030000
+
 /*
  * A "directory link" is a link to another git directory.
  *