refresh_index: remove unnecessary calls to preload_index()

With refresh_index() learning to utilize preload_index() to speed up its
operation there is no longer any benefit to having the caller preload the
index first. Remove those unneeded calls by calling read_index() instead of
the preload variant.

There is no measurable performance impact of this patch - the 2nd call to
preload_index() bails out quickly but there is no reason to call it twice.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/describe.c

@@ -629,7 +629,7 @@ int cmd_describe(int argc, const char **argv, const char *prefix)
 			struct argv_array args = ARGV_ARRAY_INIT;
 			int fd, result;
 
-			read_cache_preload(NULL);
+			read_cache();
 			refresh_index(&the_index, REFRESH_QUIET|REFRESH_UNMERGED,
 				      NULL, NULL, NULL);
 			fd = hold_locked_index(&index_lock, 0);