update-index/diff-index: use core.preloadindex to improve performance
'update-index --refresh' and 'diff-index' (without --cached) don't honor
the core.preloadindex setting yet. Porcelain commands using these (such as
git [svn] rebase) suffer from this, especially on Windows.
Use read_cache_preload to improve performance.
Additionally, in builtin/diff.c, don't preload index status if we don't
access the working copy (--cached).
Results with msysgit on WebKit repo (2GB in 200k files):