read-cache: fix wrong 'the_index' usage

We are dealing with the 'istate' index, not 'the_index'.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

read-cache.c

@@ -622,7 +622,7 @@ int add_to_index(struct index_state *istate, const char *path, struct stat *st,
 			if (*ptr == '/') {
 				struct cache_entry *foundce;
 				++ptr;
-				foundce = index_name_exists(&the_index, ce->name, ptr - ce->name, ignore_case);
+				foundce = index_name_exists(istate, ce->name, ptr - ce->name, ignore_case);
 				if (foundce) {
 					memcpy((void *)startPtr, foundce->name + (startPtr - ce->name), ptr - startPtr);
 					startPtr = ptr;