introduce CHECKOUT_INIT

Add a static initializer for struct checkout and use it throughout the
code base.  It's shorter, avoids a memset(3) call and makes sure the
base_dir member is initialized to a valid (empty) string.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1354,6 +1354,7 @@ struct checkout {
 		 not_new:1,
 		 refresh_cache:1;
 };
+#define CHECKOUT_INIT { NULL, "" }
 
 #define TEMPORARY_FILENAME_LENGTH 25
 extern int checkout_entry(struct cache_entry *ce, const struct checkout *state, char *topath);