Correct spelling of 'REUC' extension

The new dircache extension CACHE_EXT_RESOLVE_UNDO, whose value is
0x52455543, is actually the ASCII sequence 'REUC', not the ASCII
sequence 'REUN'.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

read-cache.c

@@ -26,7 +26,7 @@ static struct cache_entry *refresh_cache_entry(struct cache_entry *ce, int reall
 
 #define CACHE_EXT(s) ( (s[0]<<24)|(s[1]<<16)|(s[2]<<8)|(s[3]) )
 #define CACHE_EXT_TREE 0x54524545	/* "TREE" */
-#define CACHE_EXT_RESOLVE_UNDO 0x52455543 /* "REUN" */
+#define CACHE_EXT_RESOLVE_UNDO 0x52455543 /* "REUC" */
 
 struct index_state the_index;