update-index/diff-index: use core.preloadindex to improve performance

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):

                | update-index | diff-index | rebase
----------------+--------------+------------+---------
msysgit-v1.8.0  |       9.157s |    10.536s | 42.791s
+ preloadindex  |       9.157s |    10.536s | 28.725s
+ this patch    |       2.329s |     2.752s | 15.152s
+ fscache [1]   |       0.731s |     1.171s |  8.877s

[1] https://github.com/kblees/git/tree/kb/fscache-v3

Thanks-to: Albert Krawczyk <pro-logic@optusnet.com.au>
Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Jeff King <peff@peff.net>