refresh_index: remove unnecessary calls to preload_index()

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>